Unicode-math undefined control sequence [closed]

Question

I'm trying to get unicode-math to work. I'm not 100% what I should be doing here, but here is a minimum working example

\documentclass{article}
\usepackage{unicode-math}
\setmathfont{xits-math.otf}
\begin{document}
blah
\end{document}

If I try to run

xelatex min.tex

I get the following error:

Undefined control sequence.
l.75 ...const:Nn  \c_um_math_alphabet_name_latin_tl 
{Latin,~lowercase}

I've traced the error to unicode-math.sty which has line 75 as

\tl_const:Nn \c_um_math_alphabet_name_latin_tl {Latin,~lowercase}

I'm sure I'm compling or otherwise doing something wrong here.

Edit: Looking closely I see some warnings

LaTeX Warning: You have requested, on input line 30, version
           `2010/10/25' of package fontspec,
           but only version
           `2008/08/09 v1.18 Advanced font selection for XeLaTeX'
           is available.

LaTeX Warning: You have requested, on input line 27, version
           `2011/07/01' of package expl3,
           but only version
           `2009/08/12 v1458 L3 Experimental code bundle wrapper'
           is available.

I have tried downloading and installing new versions of these, but still get the same error