Keith - exactly the same prob with my 3i, and I thought it was window seal, so took glass out and re-sealed the rubbers. Nope.....

then thought ti was battery tray, so had it checked out.. Nope.....

Turns out to be a tiny hole less than an inch square in the rear passenger footwell, jsut where the bi that goes under the rear seat curves down and turns into the passenger footwell. Got it patched for £20 of me local auto welder bloke and no probs since then