After fixing the width to 0.95\columnwidth and tinkering yticklabels by
yticklabel style={inner sep=2pt,rotate=45,anchor=south east,font=\footnotesize}
we get

Code:
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[margin=2cm]{geometry}
\usepackage{pgfplots}
\usepackage{lipsum}
\usepackage{multicol}
\begin{document}
\section*{Introduction}
\lipsum[1]
A “balanced” risk strategy investing in a diversified range of asset classes with a return objective of UK cash deposits (LIBOR) + 3\% to 4\% p.a. over the medium term with target volatility of 6\%-8\%. The portfolio is designed to map to the ARC PCI Balanced and the STEP TMPI Medium Risk benchmarks, but at the lower end of the balanced risk range.
\begin{multicols}{2}
\section*{Risk and Reward Profile}
\lipsum[2-4]
\noindent
\begin{tikzpicture}
\begin{axis}[
width=0.95\columnwidth,
symbolic y coords={Equities, Absolute Return, Fixed Income, Property, Cash},
ytick=data,
yticklabel style={inner sep=2pt,rotate=45,anchor=south east,font=\footnotesize}
]
\addplot[xbar,fill=blue] coordinates {
(51.3,Equities) (32.1,Absolute Return) (10.2,Fixed Income) (4.0,Property) (2.5,Cash)
};
\end{axis}
\end{tikzpicture}
\lipsum[7]
\end{multicols}
\end{document}
Without rotating the labels:
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[margin=2cm]{geometry}
\usepackage{pgfplots}
\usepackage{lipsum}
\usepackage{multicol}
\begin{document}
\section*{Introduction}
\lipsum[1]
A “balanced” risk strategy investing in a diversified range of asset classes with a return objective of UK cash deposits (LIBOR) + 3\% to 4\% p.a. over the medium term with target volatility of 6\%-8\%. The portfolio is designed to map to the ARC PCI Balanced and the STEP TMPI Medium Risk benchmarks, but at the lower end of the balanced risk range.
\begin{multicols}{2}
\section*{Risk and Reward Profile}
\lipsum[2-4]
\noindent
\begin{tikzpicture}
\begin{axis}[
width=0.9\columnwidth,
symbolic y coords={Equities, Absolute Return, Fixed Income, Property, Cash},
ytick=data,
yticklabel style={font=\footnotesize}
]
\addplot[xbar,fill=blue] coordinates {
(51.3,Equities) (32.1,Absolute Return) (10.2,Fixed Income) (4.0,Property) (2.5,Cash)
};
\end{axis}
\end{tikzpicture}
\lipsum[7]
\end{multicols}
\end{document}

And without reducing the font size of ytick labels:
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[margin=2cm]{geometry}
\usepackage{pgfplots}
\usepackage{lipsum}
\usepackage{multicol}
\begin{document}
\section*{Introduction}
\lipsum[1]
A “balanced” risk strategy investing in a diversified range of asset classes with a return objective of UK cash deposits (LIBOR) + 3\% to 4\% p.a. over the medium term with target volatility of 6\%-8\%. The portfolio is designed to map to the ARC PCI Balanced and the STEP TMPI Medium Risk benchmarks, but at the lower end of the balanced risk range.
\begin{multicols}{2}
\section*{Risk and Reward Profile}
\lipsum[2-4]
\noindent
\begin{tikzpicture}
\begin{axis}[
width=\columnwidth,
symbolic y coords={Equities, Absolute Return, Fixed Income, Property, Cash},
ytick=data,
yticklabel style={text width=0.15\columnwidth,align=right}
]
\addplot[xbar,fill=blue] coordinates {
(51.3,Equities) (32.1,Absolute Return) (10.2,Fixed Income) (4.0,Property) (2.5,Cash)
};
\end{axis}
\end{tikzpicture}
\lipsum[7]
\end{multicols}
\end{document}

\documentclass{...}, the required\usepackage's,\begin{document}, and\end{document}. That may seem tedious to you, but think of the extra work it represents for TeX.SX users willing to give you a hand. Help them help you: remove that one hurdle between you and a solution to your problem. – LaRiFaRi Mar 03 '15 at 13:30width=2inor any other length you want in theaxisenvironment options. – Paul Gessler Mar 03 '15 at 13:35