You just need to get a flat head screwdriver under the washer and prise it off, they don't put up much of a fight. Then you will be able to measure it or take it to a plumbers :thumb:
Ah, sorry. I should read properly, I thought you meant the post washer. I have never changed the washer on the poppet but have wondered where I might get them from.
I've tried to replace the poppet rubbers with o-rings as I had a couple which leaked and a box of assorted sized rings from Maplin's. It didn't work and I ended up buying new poppets from Norm.
The trouble is, they aren't standard o-rings, they are square cornered L shaped rubber washers.