Aye! I could belt the beast with the persuader.. but since the pain-in-the-'arris is removing it and refitting it (and it then has to pass an MOT) I decided to fit a new one and have done with it.
I got the part from Shop4Parts and it arrived next day @ £90, as I recall.
Anyways to answer my own question for the benefit of posterity:
There is enough room between the top of the compensator and the floor of the car to get your mitts in there to unscrew the brake pipes. Once they're loose, you can unbolt the two bolts that hold the compensator to the sub-frame. If you do it the other way round, the compensator moves when you try to undo the unions.
For the sake of it, I removed the rear silencer and that gave loads of room. You could probably do the job with the silencer in place but my fella is simple to take on and off (stainless back box and clamp etc.) so it was easier than trying to work round the pipes.
The brake pipes up there are copper and tucked out of the way, so they weren't tooo corroded.. but the unions were sticking to the pipes (galvanic reaction?) and the pipes did twist a bit as I unscrewed the unions.. but some WD40 loosened it all up.
It still took some time... the biggest hassle was trying to re-fit the brake pipes as the pipes got a bit distorted and one or two of the unions then didn't line up with the holes (and of course you're on your back trying to do it back to front, with your left hand and with the thing out of sight.. or on your front with your head wedged next to the fuel tank (to get line of sight) but then with your weight resting on your gadgets...
But it went through okay.. .just I noticed how really manky the subframe is..
A job for the dry weather though...