Can it be changed?

Seems mine isn't working to seal the tank as since it got blown over by the wind it leaked down the side of the tank and stripped a nice line of paint off....

Hmmmm, seems strange. The fuel filler cap does indeed bolt in but I'd be surprised if you had damaged that as they are cast and are pretty tough. I bough a replacement ignition lock last year from flea bay and it came complete with a fuel cap and ignition barrel. Obviously a Chinese replica but may be what you need. only £14.00 the lot!

http://www.ebay.co.uk/itm/Fuel-Gas-Cap- ... SwY45UPxvs

give it a try, whats the worst that can happen apart from being £14.00 down?