So, finally managed to track down a fuel tank for the Sport. It's a Series 1 tank so took it to a local tank/radiator refurbisher and they fitted the return pipe that's missing from the S1 tanks (as my car is an S2). Got the tank back yesterday and awoke with a real spring in my step at the thought of the car being back on the road. Just before refitting it, some weird instinct made me peer down the fuel feed pipe and bingo - day light! Further investigation with a torch revealed a large hole in the pipe inside the tank near where it meets the tank wall.
One concillation is that I checked before fitting otherwise it would just have been sucking air and not fuel to the carbs!
Is this a common fault? Ahhh! Back to the tank repair garage.
Chris