Mazda 6 knocking noise from rear

[Torquey]

Hello, firstly I appologise if this is wrong forum to post this is in.

I'm desperately trying to fix a problem on a 2002 Mazda 6, causing a knocking noise on the rear passenger of my car.
Its a clunk evertime I hit a pot hole. I had something like this once before in a vectra when it needed new rear bushes.
soaking them in WD40 would quieten them down temporarily, and prove it was definately the bushes at fault.

I've searched endlessly on the web for what it could be. The best I've found is some stuff about the rear sway bar bushings.
Does anybody have a better idea of what may be causing the problem or things I can try or replace. i cant even find anywhere that would sell rear sway arm bushings.
I tried soaking everything in WD whilst bouncing up and down on the rear of the car but cannot stop the knocking

any idea's are appreciated