I've had the back end in the air and had a look, and come to a satisfactory conclusion.
On the off side, the front inner most 15mm bolt on the plate that covers the donut bush was loose and rattling around. I think it had come loose as I put a smear of copper grease on when i changed the donut bushes (in hindsight I don't know why, they shouldn't be coming off again).
There isn't a torque value for this anywhere, so when I did the job, torqued it to 65nm, the same as the centre 18mm bolt.
I retorqued all bolts on both sides, and the top mounts of rear shock absorbers.
Then I went for a spin and it seems much much better. Although not as comfortable as i would like, but I think the state of our roads is something we are all going to have to live with and get used to unfortunately.
Saying that though, if the centre bolt (18mm) through the donut bush, and the other two 15 mm bolts were torqued up nicely, I can't see how this would make that much of a difference.
Thanks everyone though for your help and suggestions
