The new gasket seems thicker than the old (this is by about 1mm!).

To address my own question, I have found online gasket with code R6346X where the part that fits in the door channel is 5.6mm (the width of the original - so should be much more secure) and there is what I can only describe as a "double edge" next to the flipper part of the gasket - this was not on the gasket I bought originally. We shall see if it is a better fit.

Clive