I have only just discovered this site. You've probably fixed it by now, but if not, I was able to get a replacement washer at a plumbers' merchant. I just took the old one as a sample, told the bloke it was from a Wirquin & he came back with something that did the job.
http://www.wirquin.com/pro-gb-produits.html was quite useful for background info, manuals etc..
Ray
August 2010