Hi Neil
I have bought some dowty washers for the same purpose, but I've not tested them in anger yet! They are ID of 11.35mm, OD of 22mm (which is probably the measurement you want) and 1.6mm thick. I think they were cheaper than the fibre equivalents and I'm sure they will work a lot better! I can send you two if you can't get them? The place I bought mine from is just around the corner from where I work and I'm going back there this week to pick up my new oil pipes they are making up for me.
Cheers, Simon