Paul,
There are a couple of places around the final drive where oil can leak. Firstly, the joint between the final drive housing and the gearbox case - there is no gasket listed for this joint, just a set of brass shims for adjustment of the mesh of the bevel gears.
Secondly, the actual final drive - the original bearing is an open type and relies on a felt seal in the final drive housing. This seal runs on a very short piece of steel tube that is pressed on to the flange of the splined shaft that carries the final drive sprocket. These short tubes are often loose on the splined shaft and sometimes get mislaid, so the seal doesn't work at all. The favoured solution in this case, is to fit a bearing with a single seal (to the outside).
On a few occasions, I have seen oil leaking from the gearbox mounting bolt if the gearbox case happens to be porous.
Hope this helps,
Regards,
Eddie.