Botty, a great question. Though they are a cheap part, I always like to find out what is the generic part that would work.
A bit of digging revealed that the drain plug o-ring is probably a "12.5 X 1.8 V75".
1.8 mm is a bit unusual cross section size for o-rings; a 1.9 mm will probably work. It's likely Viton with 75 durometer hardness.
A hardware supplier like McMaster has these:
I'm sorry that isn't much help for UK but the generic o-ring is available somewhere over there. Good luck.