I had a go at several method of stopping this as spirited driving was costing me a fortune in petrol checked on the road:

1. Cleggy's seal which improved it slightly but still left me with fuel all over the place just no quite as much as before
2. Changed the spring in the cap...again similar results
3. put a new cap on.....similar results

I had a go at all sorts and ended up with improvements each time but nothing that actually stop the fuel from pouring all over the rear wing......in the end I took drastic measures.

The fuel breathers cross over the tank and the breathing of the fuel is not affected by the filler cap I opted for a plug.

Yes a plug.

Turned aluminum (still thinking light weight..!!!) with a groove for an O ring to create a good (petrol) tight seal. I have not spilt a drop since. I know its a pain (probably) to make but I reckon on saving £££££'s just by not throwing juice away on the best twisty bits.

There is another added advantage in that it completely confuses people who dont know what it is...er like theives, or joy riders. People who might steal the car (or you petrol if the bar is shut !!!!!)....