Jacked up the rear, using the cunningly left in place tow bar bracket behind the diff
in an attempt to diagnose my "woom woom wooming" noise from the back. Chocked front wheels, engine on, in gear, set cruise (hope to god it doesn't topple off the stands and fire into the house at 50
) round the back, have a listen....
.....nothing. Not a dickybird. Right rear tyre is slightly out of round. Slightly more shushing from that disc, but other than that.... 
Wheel bearing?
...possibly. That was the suggestion last time I mentioned it. But it's quite subtle. Side load has no affect, and I think I'm right in saying that it stops while braking.
Well, by stops, I mean the brake noise drowns it out. So that gives you an idea of how subtle it is.
I'm hoping it's the audible brake "shushing" from the right rear that's signifying a warped or poorly seated disc, along with the mot advisory of uneven brake performance right rear, that's causing a noise when driving. And that's what I'm hearing.
Or it could indeed be the bearing and I'm in denial ?

I'm trying to diagnose it before Wycombe as I think Nick is offering to swap the bearing. I don't want to bother him if it's not the bearing at all
I am almost sure the noise is constant when braking and intermittent when not braking.