I can't give you a UK source, but I can tell you that I live in a place that is usually is on the lean side of 'specialty' items....despite this, a local (industrial) plumbing shop carries rolls of O-ring stock in various diameters. If I can get this stuff locally, there *MUST* be a UK source near you.
I was skeptical about making my own rings (you cut a length of stock, then use a dab of superglue to fuse the ends together in a ring). In practice, it works great! The spot where the two ends join is, in fact, stronger than the o-ring itself. Takes a bit of practice to get the right amount of glue and to line the ends up correctly.
This approach has the advantage of:
1) You get to choose the 'strength' of your o-ring (3mm, 4mm, 5mm, etc.)
2) You get to choose the 'tension' of your o-ring (by cutting to the appropriate length...some initial trial and error may be required, but o-ring stock is cheap)
3) You can always make more replacement o-rings
4) Your o-ring stock material will work for any size o-ring...so you can reuse it on a different DIY design (less waste).
Hope that helps!
