Nice to have another Griff owner here.
Mine does this too. I don't worry about it. It's got nearly 3 times the miles on yours and is a little modified...
Best guess is that the rubber seal round the filler cap (and the base of the breather/flame trap) either isn't there or doesn't work.
Never really got to the bottom of it on mine - it's annoying more than anything else - no other seals appear to be being blown.