Hadn't got anywhere with the rainforest or the bay. Mainly either the right size but in black or right colour in wrong size.
I'd seen jewellery applications before but an actual UK-based shop might be able to correctly answer whether the measurement is ID or OD. Shipping could be an issue - I'm hoping it's not 99p per o-ring! Worth an ask though.
Barnwells o-ring catalogue was quite helpful - thanks for that. Stuff like a 5% stretch being acceptable. Conveniently, this means a 12mm ID ring is exactly the right tolerance for a G1/4 thread (over the thread, not in the undercut some fittings have). Also, worth a ring to see if they do the silicone rings but other similar places only had red or black.
Thanks for the suggestions. I'll hit the phone and see what they say.