Removing gap above "Chapter X" introduces problems with the text formatting

Question

I am trying to get the page layout of my thesis sorted out. I am using fancyhdr to give me the page number in the top right and chapter name in the top left with a horizontal rule under, and sectsty to give me sans-serif font for all my section/subsection/etc headings.

I don't like the large gap above the words "Chapter X" with the horizontal rule under the header, so I'd like to reduce this. I have implemented the answer to this question: How to decrease spacing before chapter title? but I am rather unsatisfied: as you can see from my MWE below I have to redefine all of the ToC/LoF/LoT/Bibliography commands, and chapter* seems to have reverted to the serif font also.

In short, it seems very cumbersome. Is there really not a more elegant way? And if not, could someone with more knowhow than me explain how to get the chapter* heading into sffamily?

\documentclass[12pt, a4paper, oneside, fleqn]{report}

\headheight 28pt
\headsep 24pt

\usepackage{lmodern}       % gives bold italic font

\usepackage[toc]{glossaries} % add a glossary
\makeindex
\makeglossaries

%Sectioning and headings
%------------------------
\usepackage{fancyhdr}
\fancypagestyle{plain}{  %
\fancyhf{} %clear all header/footer fields
\rhead{\sffamily{\thepage}}}

\usepackage{appendix}               % allow formatting of appendix titles

\usepackage{sectsty}          % put section headings in sans-serif font
\allsectionsfont{\sffamily}
\chapterfont{\LARGE\sffamily}

\makeatletter                                   % remove hideous gap from above the chapter title
\def\@makechapterhead#1{%
  %%%%\vspace*{50\p@}% %%% removed!
  {\parindent \z@ \raggedright \normalfont
    \ifnum \c@secnumdepth >\m@ne
        \LARGE\sffamily\bfseries \@chapapp\space \thechapter
        \par\nobreak
        \vskip 20\p@
    \fi
    \interlinepenalty\@M
    \LARGE\sffamily\bfseries #1\par\nobreak
    \vskip 40\p@
  }}
\def\@makeschapterhead#1{%
  %%%%%\vspace*{50\p@}% %%% removed!
  {\parindent \z@ \raggedright
    \normalfont
    \interlinepenalty\@M
    \Huge \bfseries  #1\par\nobreak
    \vskip 40\p@
  }}
\makeatother

%reformat some of the page titles
\renewcommand{\abstractname}{\textsf{Abstract}}
\renewcommand{\appendixpagename}{\textsf{Appendices}}
\renewcommand{\bibname}{\textsf{Bibliography}}
\renewcommand{\contentsname}{\textsf{Table Of Contents}}
\renewcommand{\listfigurename}{\textsf{List of Figures}}
\renewcommand{\listtablename}{\textsf{List of Tables}}

\makeatletter
\newcommand*{\myappendixpage}{%remove page number from the Appendix page
  \begingroup
  \let\ps@plain\ps@empty
  \appendixpage
  \endgroup}
\makeatother


\begin{document}

\pagenumbering{gobble}
\pagestyle{empty}
\renewcommand{\headrulewidth}{0pt}

%title page stuff
\begin{center}
\sffamily{\Huge{\textbf{This is the full title of my Thesis}}}
\end{center}
%end title page


%This stuff at the front with roman numbering
%--------------------------------------------
\pagestyle{plain} 
\pagenumbering{roman}

\newpage
Copyright statement

\begin{abstract}
here is my abstract
\end{abstract}

\chapter*{Acknowledgements}
I'd like to acknowledge some help from some people here.

\tableofcontents
\listoffigures
\listoftables
\newglossaryentry{Term}
{
  name={Term},
  description={Description of my term}
}
\glsaddall
\printglossary[title=\textsf{Glossary}]

%Main thesis text with arabic numbering
%--------------------------------------

\pagenumbering{arabic}
\pagestyle{fancy}
\renewcommand{\chaptermark}[1]{\markboth{\sffamily{\MakeUppercase{\thechapter.\ #1}}}{}}
\lhead{\leftmark}
\chead{}
\rhead{\sffamily{\thepage}}
\renewcommand{\headrulewidth}{0.1pt}
\fancyfoot{}

\chapter{Introduction with some \textit{italic text}}
Here is my chapter

%Appendices and bibliography
%---------------------------

\appendix

\myappendixpage

\chapter{My appendix}
some stuff here


\end{document}

Answer 1

You can remove all your definitions

%reformat some of the page titles
\renewcommand{\bibname}{\textsf{Bibliography}}
\renewcommand{\contentsname}{\textsf{Table Of Contents}}
\renewcommand{\listfigurename}{\textsf{List of Figures}}
\renewcommand{\listtablename}{\textsf{List of Tables}}

and simply add \sffamily to the definition of \@makeschapterhead:

\def\@makeschapterhead#1{%
  %%%%%\vspace*{50\p@}% %%% removed!
  {\parindent \z@ \raggedright
    \normalfont
    \interlinepenalty\@M
    \Huge\sffamily\bfseries  #1\par\nobreak
    \vskip 40\p@
  }}

Some of them are not sectioning commands (abstract and \appendixpage) so you have to redefine them manually.

I would also replace \LARGE with \Huge in the definition of \@makechapterhead for consistency with starred chapters:

\def\@makechapterhead#1{%
  %%%%\vspace*{50\p@}% %%% removed!
  {\parindent \z@ \raggedright \normalfont
    \ifnum \c@secnumdepth >\m@ne
        \LARGE\sffamily\bfseries \@chapapp\space \thechapter
        \par\nobreak
        \vskip 20\p@
    \fi
    \interlinepenalty\@M
    \Huge\sffamily\bfseries #1\par\nobreak
    \vskip 40\p@
  }}

Moreover, replace the line

\printglossary[title=\textsf{Glossary}]

with simply

\printglossary

otherwise you will have the entry in the ToC in sans serif.

In the following MWE you have all starred and non-starred chapters in sans serif:

\documentclass[12pt, a4paper, oneside, fleqn]{report}

\headheight 28pt
\headsep 24pt

\usepackage{lmodern}       % gives bold italic font

\usepackage[toc]{glossaries} % add a glossary
\makeindex
\makeglossaries

%Sectioning and headings
%------------------------
\usepackage{fancyhdr}
\fancypagestyle{plain}{  %
\fancyhf{} %clear all header/footer fields
\rhead{\sffamily{\thepage}}}

\usepackage{appendix}               % allow formatting of appendix titles

\usepackage{sectsty}          % put section headings in sans-serif font
\allsectionsfont{\sffamily}
\chapterfont{\LARGE\sffamily}

\makeatletter                                   % remove hideous gap from above the chapter title
\def\@makechapterhead#1{%
  %%%%\vspace*{50\p@}% %%% removed!
  {\parindent \z@ \raggedright \normalfont
    \ifnum \c@secnumdepth >\m@ne
        \LARGE\sffamily\bfseries \@chapapp\space \thechapter
        \par\nobreak
        \vskip 20\p@
    \fi
    \interlinepenalty\@M
    \Huge\sffamily\bfseries #1\par\nobreak
    \vskip 40\p@
  }}
\def\@makeschapterhead#1{%
  %%%%%\vspace*{50\p@}% %%% removed!
  {\parindent \z@ \raggedright
    \normalfont
    \interlinepenalty\@M
    \Huge\sffamily\bfseries  #1\par\nobreak
    \vskip 40\p@
  }}
\makeatother  

%reformat some of the page titles
\renewcommand{\abstractname}{\textsf{Abstract}}
\renewcommand{\appendixpagename}{\textsf{Appendices}}

\makeatletter
\newcommand*{\myappendixpage}{%remove page number from the Appendix page
  \begingroup
  \let\ps@plain\ps@empty
  \appendixpage
  \endgroup}
\makeatother


\begin{document}

\pagenumbering{gobble}
\pagestyle{empty}
\renewcommand{\headrulewidth}{0pt}

%title page stuff
\begin{center}
\sffamily{\Huge{\textbf{This is the full title of my Thesis}}}
\end{center}
%end title page


%This stuff at the front with roman numbering
%--------------------------------------------
\pagestyle{plain}
\pagenumbering{roman}

\newpage
Copyright statement

\begin{abstract}
here is my abstract
\end{abstract}

\chapter*{Acknowledgements}
I'd like to acknowledge some help from some people here.

\tableofcontents
\listoffigures
\listoftables
\newglossaryentry{Term}
{
  name={Term},
  description={Description of my term}
}
\glsaddall
\printglossary

%Main thesis text with arabic numbering
%--------------------------------------

\pagenumbering{arabic}
\pagestyle{fancy}
\renewcommand{\chaptermark}[1]{\markboth{\sffamily{\MakeUppercase{\thechapter.\ #1}}}{}}
\lhead{\leftmark}
\chead{}
\rhead{\sffamily{\thepage}}
\renewcommand{\headrulewidth}{0.1pt}
\fancyfoot{}

\chapter{Introduction with some \textit{italic text}}
Here is my chapter

%Appendices and bibliography
%---------------------------

\appendix

\myappendixpage

\chapter{My appendix}
some stuff here


\end{document} 

Answer 2

With titlesec:

\documentclass[12pt, a4paper, oneside, fleqn]{report}

\headheight 28pt
\headsep 24pt

\usepackage{lmodern}       % gives bold italic font

\usepackage[toc]{glossaries} % add a glossary
\makeindex
\makeglossaries

%Sectioning and headings
%------------------------
\usepackage{fancyhdr}
\fancypagestyle{plain}{  %
\fancyhf{} %clear all header/footer fields
\rhead{\sffamily{\thepage}}}

\usepackage{appendix}               % allow formatting of appendix titles

\usepackage{sectsty}          % you can do this with titlesec too.
\allsectionsfont{\sffamily}
%\chapterfont{\LARGE\sffamily}

\usepackage{titlesec,showframe}
\titleformat{name=\chapter}[display]
  {\Huge\sffamily\bfseries\filright}
  {\chaptertitlename\ \thechapter.}
  {1ex}
  {}

\titleformat{name=\chapter,numberless}[display]
  {\Huge\sffamily\bfseries\filright}
  {}
  {0pt}
  {}

\titlespacing*{name=\chapter}{0pt}{-20pt}{40pt} 
\titlespacing*{name=\chapter,numberless}{0pt}{-40pt}{40pt}


%reformat some of the page titles
\renewcommand{\abstractname}{\textsf{Abstract}}
\renewcommand{\appendixpagename}{\textsf{Appendices}}

\makeatletter
\newcommand*{\myappendixpage}{%remove page number from the Appendix page
  \begingroup
  \let\ps@plain\ps@empty
  \appendixpage
  \endgroup}
\makeatother


\begin{document}

\pagenumbering{gobble}
\pagestyle{empty}
\renewcommand{\headrulewidth}{0pt}

%title page stuff
\begin{center}
\sffamily{\Huge{\textbf{This is the full title of my Thesis}}}
\end{center}
%end title page


%This stuff at the front with roman numbering
%--------------------------------------------
\pagestyle{plain}
\pagenumbering{roman}

\newpage
Copyright statement

\begin{abstract}
here is my abstract
\end{abstract}

\chapter*{Acknowledgements}
I'd like to acknowledge some help from some people here.

\tableofcontents
\listoffigures
\listoftables
\newglossaryentry{Term}
{
  name={Term},
  description={Description of my term}
}
\glsaddall
\printglossary

%Main thesis text with arabic numbering
%--------------------------------------

\pagenumbering{arabic}
\pagestyle{fancy}
\renewcommand{\chaptermark}[1]{\markboth{\sffamily{\MakeUppercase{\thechapter.\ #1}}}{}}
\lhead{\leftmark}
\chead{}
\rhead{\sffamily{\thepage}}
\renewcommand{\headrulewidth}{0.1pt}
\fancyfoot{}

\chapter{Introduction with some \textit{italic text}}
Here is my chapter

%Appendices and bibliography
%---------------------------

\appendix

\myappendixpage

\chapter{My appendix}
some stuff here


\end{document}