Had a day out on the banks of Loch Awe. On the trip there I became aware of a new noise; a sort of muted howl, unable to determine front or rear, drivers side or passengers side. Noise the same regardless of gear or on the overrun, so unlikely to be gearbox.
Coming home the noise returned, this time with a slight 'buzz' through the steering wheel. Both noise and 'buzz' goes away on left hand bends, returning on the straight and stays on right handers, so (hopefully) not the haldex or diff.
Looks like a front wheel bearing on the way out.

Only done 237,357 miles, dammit.