I got the opportunity to check Blobbish's car today , his cap is different , was much tighter than mine and the rubber washer looks thicker, also the edge of the top of the tank has a wider shoulder meaning greater surface area for the washer to seal
My car has an early kit around three years old and has done about 30,000 miles , his kit is new
It could be that the cap has been over tightened in the past and the thread is damaged.
I have ordered some nitrile sheet so will fabricate some new thicker washers and see if that helps
