Leaky fuel cap
 

every time I fill up, I get covered in unleaded when I accelerate and It's coming out of the top of the filler cap.
Is it as simple as replacing the 'o' ring, or (as is more likely) replacing the whole unit?
TIA
Nige

only time thats happened to me its not been shut properly

The breather holes could be blocked? They corrode up on the SS/Monsters.
There are also hoses that connect to tem inside the tank that act as drains. Check that they are in place and also that they are not blocked or trapped.