I wanted to put my data in a multi column and multi row table, whilst aligning the entries by decimal point. For that I am using the package siunitx. There problem lies within the formatting of the three sub columns: 'Position 1', 'Position 2' and 'Position 3'. I do not mind whether the entries are centred or positioned to the left, but at the moment they are not the same and I cannot understand why. In addition to this in column 'Position 1', there are spaces between the 3 s.f. values and the \plusminus, have tried to get rid of this using ~ but to no avail. If you can help in any way I would be so, so grateful! Many thanks.
\documentclass[12pt, oneside]{article}
\usepackage{geometry}
\geometry{a4paper}
\usepackage[parfill]{parskip}
\usepackage{graphicx}
\usepackage{amssymb}
\usepackage{titling}
\pagenumbering{arabic}
\usepackage{wrapfig}
\usepackage[nottoc]{tocbibind}
\usepackage[round,authoryear]{natbib}
\usepackage{placeins}
\usepackage{gensymb}
\usepackage{array}
\usepackage{booktabs}
\usepackage{amsmath}
\graphicspath{ {paperfigures/} }
\usepackage{textcomp}
\usepackage[font={footnotesize}, margin=2cm]{caption}
\numberwithin{equation}{section}
\usepackage{tabularx}
\usepackage{siunitx}
\usepackage{floatrow}
\usepackage[font={scriptsize}, margin=0.5cm]{subfig}
\usepackage{multirow}
\begin{document}
\begin{table}[]
\scriptsize
\centering
\caption{Results for testing the directionality of the Light Diode, when detecting the light intensity at three locations in the room, at three different heights and four different orientations where 0\degree~indicates the diode is pointing directly upward, and each other degree value is taken from this.}
\label{tab:directional}
\sisetup{output-decimal-marker = {\cdot}}
\begin{tabular}{ll|S|S|S[table-format=-2.1]|}
\cline{3-5}
&
& \multicolumn{3}{c|}{\textbf{Mean Light Intensity at positions
(mV)}} \\
\hline
\multicolumn{1}{|l|}{\textbf{Vertical Position}} &
\multicolumn{1}{c|}{\textbf{Orientation}} & \multicolumn{1}{c|}
{\textbf{Position 1}} & \multicolumn{1}{c|}{\textbf{Position 2}} &
\multicolumn{1}{c|}{\textbf{Position 3}} \\ \hline
\multicolumn{1}{|l|}{\multirow{4}{*}{10 cm from ceiling}} & 0$\degree$
& 2.3 $\pm$ 2.0 & 5.8 $\pm$ 2.0
& 8.2 $\pm$ 2.0 \\ \cline{2-5}
\multicolumn{1}{|l|}{} & 45$\degree$
& 3.7 $\pm$ 2.0 & 2.9 $\pm$ 2.0
& 26.1 $\pm$ 2.1 \\ \cline{2-5}
\multicolumn{1}{|l|}{} & 90$\degree$
& 12.8 $\pm$ 2.0 & 4.5 $\pm$ 2.0
& 13.6 $\pm$ 2.0 \\ \cline{2-5}
\multicolumn{1}{|l|}{} &
180$\degree$ & 1.9 $\pm$ 2.0
& 4.0 $\pm$ 2.0 & 3.0 $\pm$ 2.0
\\ \hline
\multicolumn{1}{|l|}{\multirow{4}{*}{Centre}} & 0$\degree$
& 10.4 $\pm$ 2.0 & 8.8 $\pm$ 2.0
& 11.4 $\pm$ 2.0 \\ \cline{2-5}
\multicolumn{1}{|l|}{} & 45$\degree$
& 12.5 $\pm$ 2.0 & 8.0 $\pm$ 2.0
& 7.2 $\pm$ 2.0 \\ \cline{2-5}
\multicolumn{1}{|l|}{} & 90$\degree$
& 5.3 $\pm$ 2.0 & 2.9 $\pm$ 2.0
& 5.1 $\pm$ 2.0 \\ \cline{2-5}
\multicolumn{1}{|l|}{} &
180$\degree$ & 1.8 $\pm$ 2.0
& 1.4 $\pm$ 2.0 & 1.2 $\pm$ 2.0
\\ \hline
\multicolumn{1}{|l|}{\multirow{4}{*}{10 cm from floor}} & 0$\degree$
& 9.6 $\pm$ 2.0 & 6.9 $\pm$ 2.0
& 11.0 $\pm$ 2.0 \\ \cline{2-5}
\multicolumn{1}{|l|}{} & 45$\degree$
& 12.2 $\pm$ 2.0 & 9.0 $\pm$ 2.0
& 3.5 $\pm$ 2.0 \\ \cline{2-5}
\multicolumn{1}{|l|}{} & 90$\degree$
& 2.3 $\pm$ 2.0 & 2.2 $\pm$ 2.0
& 1.5 $\pm$ 2.0 \\ \cline{2-5}
\multicolumn{1}{|l|}{} &
180$\degree$ & 2.1 $\pm$ 2.0
& 1.6 $\pm$ 2.0 & 0.9 $\pm$ 2.0
\\ \hline
\end{tabular}
\end{table}
\end{document}
