Won't win concours with a dirty cap !
These fuel caps are made by Newton Automotive, tracking down the locking inserts on the Car Builder Solutions web site, they are available as non-venting, and as venting.
In the case of emission control they need to be non venting inserts so the carbon filter stores the fumes.
For older vehicles they would normally be vented inserts. In that case the large disc and o-ring would become the primary seal and release fumes once the pressure is high enough to lift the disc.
The Newton locking inserts come in two styles, the older type has a sort of rubber band style of gasket
The un-vented
https://www.carbuildersolutions.com/uk/un-vented-lockable-insert-for-aston-necks-excl-2The vented
https://www.carbuildersolutions.com/uk/vented-lockable-insert-for-aston-necks-excl-2I have purchased the non-venting and it did fit my old filler cap.
The later (current) style of locking insert has an o-ring seal, the part number on the insert here on my desk, for the non-vented is #Z45LT. I haven't yet found these on a re-sellers site, There was a thread some time ago on Talkmorgan where someone got hold of Newton and purchased an insert.
Assuming your cap is in good shape, it may be worth getting a new locking insert as this should be the primary seal for a non-vented system.
Another thing to check would be the fuel hoses to see if there are any cracks. My old hose looked OK but I did find some fractures although they were under the hose clips.