Mick,
This is a topic that came up on another forum recently. Someone advised to completely remove the stuff that's stuck in the tank lid - then what you do is get some high modulus silicone sealant and put a big bead of it round the top edges of the tank instead. Obviously would need a good old clean beforehand.
Apparently this lasts ages, much longer than keeping on replacing the normal seals. Reckon I'll give it a crack myself when mine gets a bit more ragged!