Thought I'd give the conclusion to this story.
B&Q eventually replied to my emails.
The "manufacturer" of the shower (importer?) turned out to be PJH Group, based in Manchester.
http://www.pjhgroup.com/products/private-label
I still wanted B&Q to either send me the correct part, or...
Thanks - I sent the first email several weeks ago, before the outage mentioned on their web site, and a second a few days ago, no reply to either.
With the phone you put someone on the spot, they're very unlikely to know the answer. With email they have the chance to look into it and then...
http://www.diy.com/diy/jsp/bq/nav.jsp?isSearch=true&fh_search=03636053&x=0&y=0
If the link doesn't work, it's the B&Q Deco Mixer Shower in chrome, EAN 03636053.
It's dripping so I probably need a new cartridge or service kit or whatever.
Have emailed B&Q, quoting the EAN number but no...