Page displays as junk code

I’m having problems with pages on my site (http://www.mattnolanphotography.co.uk) displaying what looks like junk code when the cacheing is turned on.

I’ve uploaded a screenshot to Dropbox of the sort of thing I’m seeing.

I’m baffled. Has anyone else seen this, or has any idea what I can do?

Thanks in advance.

http://wordpress.org/extend/plugins/w3-total-cache/