TD5 110 transfer box oil leak

Looking for some pointers, I appear to have a large oil leak coming from the centre of my 110, centre exhaust silencer covered. I have had a brief look underneath and it appears to be coming from the transfer box. Can anyone tell me what seal/s may be the usual suspects and whether the box needs to be removed for replacement. (assuming it is the transfer box!).
Also a price guide would be handy! Cheers.

clean white tissue and wipe it.

transfer will be ep90 (gold or maybe dark brown if old)
gearbox will be red.

see if you can follow the leak up.