At first glance, I thought it was an original BSA tank, but now that I've had some time to think about it, I think that is a Rickman tank, or possibly a tank from a Manx Norton.
Check out these pages for photos of similar tanks:
http://www2.cnet-machida.ne.jp/tankya/1tanks/SRtank-main/norton.htm
http://www.freewebs.com/norton/
http://www.classic-motorcycle-fiberglass.com/tanks/Norton%20Manx%205gal%20Fuel%20Tank.htm
http://www.classic-motorcycle-fiberglass.com/commando.htm
Not-too-well educated guesses, I'm afraid; never actually owned any Brit iron, so I can't be positive about identifying the tank.
Hope others can help you pin it down... Lastly, I'm pretty sure it ISN'T a stock Triumph tank.
Good luck; let us know if you ever make a positive ID for it.
EDIT: Had to modify one link, because it was broken the way I originally posted it.