I have a limited number of these available as there are not many cars that use these! This is a set of replacement gaskets for the large filler cap used on early fixed heads only.
There is a five inch round foam (with half a dozen holes) that goes under the plastic cap and a 2.5 inch seal (C profile) to go next to the filler pipe on the spring loaded part of the cap. Fitting is not too bad, but you need a good cross head driver as the screws are at an angle
Also on ebay, but pm me directly. £12.50 +postage.