To create a random image in our design I have created a nested content area in the main design parse file, then created a customisation to use a random asset list I created (this worked BTW Avi, thanks). When I apply the customisation to a page all the links to the image and css files in the main design are broken. Do I need to link all these files to the customisation as well as the main design? I thought they inherited, they're all linked in the parse file as location=mysource_files/xxx.xxx and it is being applied.
They should all be inherited, yes. What URLs is Matrix trying to paint for those assets?
eg: Parse file - img src="mysource_files/hotlink_premier.jpg" renders as img src="http://192.168.11.146/?a=2108" for pages with main design applied (its not yet production) and renders as img src="java script: alert(js_translate('no_url_found', 'hotlink_premier.jpg', 'CRC Main Design - Customisation'));" in the pages with customised design.
Should I just junk the customisation and try again?
Either way, the URLs are incorrect. Check the URLs screen of the Designs folder asset and make sure that your main URL is applied to that (so that Matrix can create suitable URLs for the images).
Cheers once again Ari. 
Design URL was set to localhost.