I had the same problem last week, it turned out not to be a gear box leak at all, but a diesel leak from the fuel preasure regulator. It drips onto the right hand side of the gear box, runs down the side and looks just like a gear box leak. have you had a look underneath? is it only wet down the drivers side of the box? if so its more likely to be diesel.
Definitely Oil as apposed to Derv, It seems to be dripping from what I assume is the draining plug for the G/Box oil...Hmm Possibly the washer/bolt then