It sounds like you might be having the same problem I had last year.
The sealing/locking mechanism has become worn and it is only a matter of time before the cap stops making a seal and you'll start losing petrol under braking.
A brand new cap (cap only, not the surround that's fixed to the tank) complete with a lock that would still opererate with the original key cost me roughly 35 notes from HD. (I had to supply the VIN to get the key match)
You can get one from the firm that provided Buell with the caps for the S1 for about 20 quid more, but it probably won't have 'Buell' written on it, if that bothers you. http://www.newton-equipment.com/
Please note, these guestimates are based on last year's prices, so expect one hell of a price hike.
EDIT: Come to think of it, I was prepared to pay an extra twenty quid for the Newton item, over the cost of one from HD.
The fact that I ended up getting Harley's quality Taiwanese product must mean that East End cratmanship must have cost between 60 and 70 notes.
Shame on me
