Propshaft 'clunk'.

[M3 Compact]

<40K miles 2009 3.0 Tdi AUTO has a 'clunk' when changing F/R direction and taking up the drive. Also very occasionally when changing gear.

We've had it on the ramp; it's easy to replicate the noise by gently taking up the drive with the footbrake applied and I think my initial suspicions are incorrect.

It sounds from inside to be coming from around the centre of the car and I was expecting to find a failing propshaft centre bearing, but despite being able to 'feel the noise' while holding the centre bearing housing, there is visible rotational movement in the propshaft rear joint where it connects to the diff pinion flange.
It's as if there's some 'slack' in the rear joint/coupling.

Google hasn't been much help. So:

Is this a common issue with Quattro propshafts?
Audi only sells the complete shaft at £1100, does anybody know where I could buy just the joint?
Is the joint/coupling likely to be used in other models? (Wifey has a B7 A4 3.2 Quattro MANUAL vert and I could try swapping the joint over.)
Any links or alternative resources that might be helpful?
Or any thoughts/suggestions at all?

Thanks in advance.