I'm using "multirow" in a table but cannot get the columns to centre properly. Can anybody help? My code is below along with a screenshot of the table it's giving me.

Thank you!

\begin{table}[ht]{}
\caption{Results of Conceptual Design}
\centering
\def\arraystretch{1.5}
\hline\hline
\multirow{2}{*}{\raisebox{-\heavyrulewidth}{Material}} & \multicolumn{3}{c}
{Thickness Breakdown (\%)} &  \multirow{2}{*}{\raisebox{-\heavyrulewidth}
{Total Thickness (mm)}} & \multirow{2}{*}{\raisebox{-\heavyrulewidth}{Lost
Profit (\pounds)}}\\
\cmidrule{2-4}
& 0\degree & 45\degree & 90\degree & \\
\hline
a &  1 &  1 & 1 & 1 & 1 \\
b & 1 &  1 & 1 & 1 & 1  \\ [1ex]
\hline
\end{tabular}
\label{table:conresults}
\end{table}


something like this?

\documentclass{article}
\usepackage{siunitx}
\usepackage{booktabs, makecell, multirow}

\begin{document}
\begin{table}[ht]
\caption{Results of Conceptual Design}
\label{table:conresults}
\centering
\setlength\tabcolsep{12pt}
\begin{tabular}{c ccc cc}
\toprule
\multirow{3}{*}[3pt]{Material}

• use of multirow hasn't anything with column centering. your problem is to long multicolumn header so the last column under it is wider as is its natural width