If it has anti roll bar drop links it will probably be those.

Just one side? Knackered dampers can cause knocking. Have a look under the wheel arch above the back wheel. Is the bump-stop and dust cover intact? Bump-stops can break so that dust cover drops and the shiny rod of the damper is exposed. Then you get muck going through the top seal and the damper fails. Easiest way to check the damper is to jack up, remove wheel and take the bottom nut (or bolt) off the damper, then try pushing the damper body up and down. If it's bad, it'll move easily by hand. Dampers can be easy to change if the car is properly designed, but then your car is French.

It could also be a broken road spring. A common problem these days.

I'd suggest going to your local MoT only tester (ie somewhere that does no servicing or repairs) and ask them to inspect your rear suspension for you on their ramp. This may cost you 20 quid, but it's money well spent if it diagnoses the problem and saves you changing stuff by guesswork.

Of course, if you have the means to inspect it yourself and can find the problem, then great.

Drop links (ball joints), shocks, broken springs and worn bushes can all knock.