Finally got this cured (No. 2) - thanks to Gary at Bigbury 4x4 in Canterbury.
I've had loads of suggestions, from downpipe to counter-weights and so on, but the cure was the handbrake unit.
Apparently, there was a recall on Disco TD5's back in the late 90's, where a certain piece was removed from the handbrake unit as it was causing problems - Gary checked this, and found that not only that the handbrake unit was full of gunk, but also required a modicum of adjustment to ensure that the fittings were tight. This explained that when the fault was being replicated (whilst stationary), that it could be relieved by gently pulling up on the handbrake whilst holding it in the offending rev-range.
No parts required, just a bit of adjustment and clearing out of some crap, and totally thrilled that that fecking rattle has finally GONE! Oh, the relief..... what a good LR mechanic - can't recommend him highly enough.