Felt it is about time for me to contribute, rather than just reading talkmorgan!
I bought my 2011 +4, 4 seater 5 years ago and it always sagged on the driver's side. This got progressively worse, eventually becoming unacceptable over the next couple of years. I didn't want to go to MMC for new springs as they would have supplied some standard replacement from whoever their cheapest supplier was at the time. I went to Peter Ballard of SSL with a carefully worded specification. In short it stated that the springs were to give a more compliant ride and that the car was to have equal ride heights across the rear!
The first couple of sets of springs SSL supplied (from Owens, if I remember correctly) didn't crack the problem, the car still sagged.
Peter came to my house and put the car on load cells which showed that with no persons on board it is about 60lbs heavier on the driver's side than the passenger side, If one looks carefully at the Duratec engine it is apparent that it is mounted very slightly over to the driver's side (RHS). In addition there is all the weight of the exhaust on that side as well.
THIS WHY PLUS 4 REAR ENDS SAG TO THE RIGHT.
So Peter had a special set of springs made for me that have the same rate but different starting cambers - the greater camber being for the driver's side.
I put these in about 3 years ago and the car is still level and doesn't drag its rear end over every speed hump, or matchstick for that matter and the ride is much improved.
I have to give SSL due credit they stuck with the problem until a resolution was found and I would thoroughly recommend them. Over this period I did become very adept at changing rear springs!
