[quote author=billythekid link=1126513405/0#4 date=1127842860]take off the case, the control knobs are often integrated into the cover. There will be a way of "pulling" it off once the cover is removed. Check your local plumbing supplies place for an O ring to fit.
btk [/quote]BTK you are the Dogs Whatsits. Did as suggested then called in to an old fashioned style Ironmongers come Hardware shop [Carwrights on the outskirts of Falkirk] - you know loads of stock and knowledgeable staff. I asked for a packet of mixed sizes but the guy said "oh we sell them individually so that you get the size you need" - I nearly fainted clean away

Had to buy a few different sizes though as I had left the sample behind