You can use titlesec and do this inside a group so that changes are applied only to a chapter
\bgroup
\titleformat{\chapter}{\filcenter}{}{0pt}{\normalfont\large\bfseries} %% remove \filcenter if not needed
\titlespacing*{\chapter}{0pt}{0pt}{20pt}
\chapter*{Statement of authorship}
\egroup
If the chapter comes at the end, no need of \egroup and \bgroup. Also \filcenter puts the chapter at the center. Change \large to whatever you like.
\documentclass[11pt]{report}
\usepackage{titlesec}
\begin{document}
\chapter{Some chapter}
\bgroup
\titleformat{\chapter}{\filcenter}{}{0pt}{\normalfont\large\bfseries} %% remove \filcenter if not needed
\titlespacing*{\chapter}{0pt}{0pt}{20pt}
\chapter*{Statement of authorship}
\egroup
\chapter{Some chapter}
\end{document}
