Hey Tom, although everything you're proposing below in regards to SEO is great, and should be done anyway, I don't think that's the reason you're seeing poor (or no) performance in Google Shopping.
Disclaimer, my knowledge of Google Product Search is limited to my experience in the U.S. There may be a whole different set of requirements for the UK.
Over the last few months Google Product Search has been rolling out additional requirements and perhaps their most recent update(s) affected you. I can't say for sure, but I'd make sure you are providing every attribute possible that they request, even the 'suggested' attributes. Additionally, make sure you are not providing more than one record for each Unique Product Identifier. Glancing at your site, I could not find any product SKU / UPC, etc. associated with your products, but that doesn't mean you're not providing it to Google.
See here the the most recent update: Google Products Search Feed Specs/Policy Changes
Hope this helps.