The challenge was to find the right combination of outside/inside diameter, and not being too squidgy. I failed to find the appropriate size in my scraps of tubing, ranging from ballpoint pen refill to hosepipe. So I tried our local car spares shop, DIY store, plumber, modelmaking supplier, aquatics/garden centre, chemist, pub/home-brewing....and eventually extensively trawled Ebay. So I have two packages arriving later this week, and will see which fits best.
