Well, I managed to fill it through the breather.
Didn't take as long as I expected.

I've got one of those hand pumps pictured above, but the rubber ring seems to have shrunk back, and it no longer seals against the body of the pump.
So I extended the hose on it, and used it like a funnel from inside the wheel arch.

As for the front diff drain plug, I was beginning to think that even a lever that could shift the earth still wouldn't be long enough.
Once I found some flat steel that fitted the slot properly, it eventually moved.

So success on both fronts (or is that both front and rear?).

Thanks for the suggestions, everyone.

Next problem is that I can't budge the filler on the transfer case. It is beginning to round off, so I think I'll just leave it alone.

Ian