"can I remove just the transfer box and replace all seals etc or must the auto box come out as well?
All this assumes that the Td5 is mostly the same as a Tdi D1, (same transfer case), I have no experience with D2's.
Just remove the transfer case on it's own, I did mine unassisted, (fitted to 300Tdi manual), through the top, removed the transmission tunnel console and the rubber infill section and made a small lifting gantry device that uses an M16 "all thread" bar to do the lifting, mounted through a bar and a pair of legs sitting on the front floor of our Disco, the whole job carried out in my front yard, (I do not have a workshop or even a concrete slab to work on), which made it a bit more difficult, I am retired so not as strong as I used to be, but still, my point is it's doable without removing the gearbox and the threaded bar does all the lifting you just need to turn the nut with a spanner.
You will find that most, if not all the oil will be leaking from the two intermediate shaft "O" rings, I replaced them plus the input shaft seal on my transfer case that had done 300,000 klms, no other work done, replaced it in the Disco and it has not leaked one drop since the re-sealing exercise, well worth the two days work.