Well after a bit of a struggle trying to undo the drain plug - spent half the day with that; ended up needing a heavy duty hex socket set and 30inch breaker bar! It seems that a previous attempt was made by a cold chisel and hammer and the gouge had gone through the mating surface of the plug and even dented the copper washer. When I finally did manage to remove it, there was no swarf as such on the plug magnet, just a bit of sludge.
I seem to have somehow managed to take the picture so you can't see the damage to the plug/washer in the pic - even now I'm not quite sure how I managed that!
The level wasn't as low as I had initially thought - there was nearly 1 litre in there, out of a total capacity of 1.1 litres. The oil wasn't that bad either - it was Millers fully synth stuff and again, hadn't deteriorated as much as I initially led you to believe. The pic doesn't show it but the oil was still quite translucent and greenish-yellow.
.
I have a suspicion that the leak maybe at least in part, caused by the drain plug not seating 100% due to the damage to the mating surface and washer. However would that explain how the oil covers pretty much the whole of the bottom of the diff?
Anyway, I will order a new drain plug and washer on Monday and fit it with some new oil and keep a close eye on it. I checked the top of the breather tube from the engine bay and that seems OK. What is the best way to access/check in at the diff end?
Looking further back under the truck, I then noticed this......
I feel a whole new thread coming on.......
Thanks again for you help and support.