I have an Armitage Shanks AMSTD FASTPART flush valve for toilet and require a rubber seal washer to replace mine which is leaking, the dimensions are OD 80mm ID 56mm and thickness 3mm.I have been unable to find one after looking everywhere. Any advice on where I could get one would be appreciated. I was wondering whether you can cut one out which would do the job from 3mm neoprene sheet?