Right,
Remove the parcel shelf and supports.
Get someone to bounce the back of the car whilst you look at the top mounts of the rear suspension. There should be no movement from the tops whatsoever. However from what you have said it seems likely that the little circlip on the piston has broken off. This stops the spring compressing right down. If it has broken then the top of the piston will start coming through the top mount hole.
If it is this then you need to get yourself down to GSF, Rear Gas shocks are £40 a pop, Top mounts are £5.
The work is easy to do.
Jack the rear of the car up completely, unscrew the bolt at the top of the suspension and the entire rear axle will drop down. Then follow the strut down onto the lower part of the axle and remove the bolt that screws through the bottom and next to the brakes. This will free up the strut and you will be able to take it out easily.
Stick the spring on the new shock as you found it on the old one. Replace in same way as you took out, lower car back down and it shouldn't bounce anymore.
Hey presto £85-90 spent and an entire job done by yourself, you will be a regualr mechanic in no time.