Vauxhall Corsa suspension sounding bad?

My car is fine when im on my own, but whenever i have 2 or mroe people in the car, if i go over slight bumps, or speedbumps, or anything that would make the back of the car jolt down a bit, there is a fairly loud clunk noise. Sounds like 2 bits of metal coming into contact with eachother.

My m8 has a corsa, same model as mine, we both got the 1.0 litre 12V, corsa trip. mine is 1999 T reg, his is 2000 V reg. his doesn't make this noise, and compared to his mine feels quite wobbly and jelly like when turnin etc.

I am only 17, first car, don't have much experiece with cars and their performance yet so any suggestions would be appreciated.
Obviously i want to get rid of this problem as i dont like havin a car full of m8s and every1 thinkin my car is fallin apart! xD

If its just a case of suspension being hardened, any ideas on how much this could cost?

Thanks in advance.