Gareth, thought mine was leaking from the oil seal. Like F355GTS, it turned out to be the gearbox, it wasn't obviously gearbox oil because it was such a small amount everytime (and wasn't filled with Redline MTL), but enough to cover the underside from the bellhousing backwards. Unfortunately the only way to find out for sure was to remove the engine, I just did the engine to start with but wished I'd done the gearbox/bellhousing/engine all in one go.