I met a tame plumber in Grahams yesterday who told me the reason I couldn't find the correct washer for my gas fire connection was because it shouldn't have one! The reason being, if it crumbled, as the original one did, it could leak.
It transpires that the restrictor pipe had an incorrect fitting on the end of it, an iron elbow, which was why the female nut on the fire wouldn't tighten up onto it without a washer. He told me "I shouldn't say this but.....use an 8mm compression elbow instead".
Now sorted :)
Si