There is a non-trivial oil leak in the differential of my Mk I. It appears to be coming from the front where the drive shaft is attached. Is there a seal there? And if so, can it be repaired by removing the drive shaft rather than removing the whole rear axle?
Yes
Yes but with provisos. There should be numerous posts regarding front diff seals in the archives, use the spyglass (top right) and search.
Not trying to be awkward but there are screeds already written on the subject.