knock back left corner
 

I have a knock coming from the rear of the car on the passenger side, its more obvious when going over a bump and i have noticed that the tyre is worn more on the outside edge than the inner. I have taken a look and it doesn't look like anything is broken so i think it might be one of the linkages or bushes.
Whats the best place to start?

what mondeo mk3 have rear subframe bushes that unbond and frame knocks on body , and the normal roll bar linkage and bushes that wear or a snapped spring

yes its a mk3, 2006 ghia.

Is there anyway to check what part it is?

Had the same problem on my 2004 tdci 130 ghia. The anti roll bar bushes were shot, so were the anti roll bar drop links so I thought I would go the whole hog and polybush it throughout. Nearly all done just the offside subframe bushes to replace now with the poly ones. Also check the handbrake cable. It should clip on to the rear tie bar. Ive heard of this clip braking and giving a knocking noise as the metal outer cable knocks on the tiebar. Easily fixed with a cabletie apparently. Hope this helps. Good luck. Mark