Hi woodsie, yes I've had the same problem as you and it's due to dirt build up around the
fuel filler neck and the rubber on the cap. I just took the cap off and soaked it in paraffin for half an hour while I cleaned around the filler neck with a paraffin rag you'll be amazed at how much muck comes off.

Then take your filler cap out of the paraffin and give it a good shake and wipe it dry and leave it to dry out. Get a small jar of petroleum jelly and smear a little on the cap a wipe off and run some around the filler neck on a rag, be careful you don't cut your finger in doing so. That's it jobs a good one. :thumbs: