Hi Mike,
What a nightmare!! I know the problem you have with the seat as I had exactly the same problem with my drivers seat /ubbthreads/images/graemlins/burningmad.gif It drove me nuts for months.. It would rock back and forth going over bumpy roads or under heavy braking.
The problem I had was with the seat base. It had warped, and was therefore not sitting flush with the catches and hence had about (as you say) 10mm play back and forth in it.
It took the stealers 3 yes three! attempts to resolve the problem /ubbthreads/images/graemlins/burningmad.gif Eventually they got it sorted by replacing the whole seat base, fortunately for me all work carried out under warranty.
Hope this helps
Ben /ubbthreads/images/graemlins/beerchug.gif