Right, this is how im going to do it.

Wall off
Pack out voids between studs with 50mm 140KG dense fibre matting
Fit resillient bars across studs to decouple (as per Nick says)
Lay 1 layer of 12.5mm Fermacell (better than soundbloc pb, and more expensive!)
Apply 2.5mm specialist soundproof membrane (how expensive!)
Lay a second layer of 12.5mm Fermacell

Job done!

If I hear anything through that, he can wank me off with his arse!