Yeah -thanks pj123, great thread!
Well, rear diff oil was changed this morning. Only challenge was getting the 17mm hex socket into position to get the fill hole plug out. It's really cramped in there, but after a bit of gentle "persuasion" and swearing it went in OK. As expected, the magnetic drain plug had captured some metal filings over the years but I cleaned these off before re-filling the diff. The oil that came out was blackened but OK. Re-filled with 1L of Fuchs Titan Sintopoid 75W85 gear oil bought from Opie oils, using a Clarke oil suction pump. Took about an hour to do all-in.
The problem came with the front gearbox - both the 14mm drain and fill hole plugs are security-hexes on my 2003 model (ie a 14mm hex bit with a protrusion in the centre) which means I couldn't get them out with my standard 14mm hex socket. Does anyone know where I can get a 14mm hex-socket with a dimple/hole in the centre? It's really annoying because the transmission drain plug and the rear diff plugs are just standard hexes, so why they've used security-hexes on the gearbox I don't know! Thankfully I was only replacing the oil as a service, not to correct any problem with gear-changing, so at least the car is still useable.
Anyway, I decided to do the N249 bypass instead, which I've been meaning to do for some time.