Ok, here comes my problem.

I have a 2005 Mini 1 with standard suspension,which is not level on the rear, bought the car 3 months ago and it didn't seem as bad as it is now.

Jack the Car up & it's fine, both wheels drop the same amount.
Springs checked out - all OK, no cracks or breaks.
Dampers have no leakage and feel the same either side, so I doubt the internal seals are passing on 1 side.

I've read about the car having to be bolted up with a set weight in the car and was thnking if the previous owner did some work and didn't bolt it up correctly.

Anyone had any similar problems?

Cheers
Kidology

Extra things to check:

Are the trailing arms bent?
Do the front mounts for the trailing arms look to have been disturbed?
Are the camber adjusters (if fitted) set the same side to side?