Every now and again, my ford focus makes a loud clunk noise from the back of the car.

I think it happens more when i go round corners, but cant be to sure.

Any ideas?

check your coilsprings

yeah as ste said i would check your suspension, and exhaust for movement.. its quiet common for a loose exhaust system to bang at somepoints.

rear anti roll bar link?

i have noticed mine doing this aswell, only started noticing last night, rear drivers side. going to take a look under it tonight