Hi Bill, thanks for your response. My mechanic checked the shocks and springs and they seemed to be in good condition. He could replicate the creak by bouncing the rear when it was raised on a lift but didn't seem to think the shocks were the problem. :-/

The noise started about 10 days after I jacked the car up to change the front brake pads but I'm pretty sure that's totally coincidental as the rear wheels were only off the ground for a minute or two as a result of the front jacking point being used to lift the car high enough before I lowered it onto axle stands.