Thanks for the replies, I've finally worked it out with your help.
I was thinking the flap was more complicated that it actually is. I thought the flap was held shut even when the car was unlocked, and to release it you had to push on the cap to allow it to be released.
I had another look at it, and yes it is the tether putting pressure on the flap when it is closed.

Maybe it has got less pliable with age, but it loops as the flap is closed, and I found if it loops downwards, it puts pressure on the flap, but if it loops upwards it doesn't, I'll just have to make sure it loops up whenever I go to the Petrol Station.

Cheers
Terry

ps. to all you northern hemisphere Roadster drivers, tomorrow it is going to be 86F, heading up to 95F by Sunday, guess who will have the roof down the whole time?
Sorry to tell you that!!