Ah well, just for info, I found the problem..
Removed the seat and had a good look around at the mechanism, plus much googling on parts sites.
There is a service kit for the seat hinge mechanism. Relates to this:
https://www.ebay.com/itm/VOLVO-S40-S...2279828?_ul=AR
VOLVO SERVICE KIT FOR SEATBACK WHIPLASH PROTECTION SYSTEM. THESE ARE MADE TO BEND DURING REAR END COLLISION !!
One of them has definitely been bent and can see a stress fracture.. Probably related to the rear end shunt the car had end of last year!! Not something the insurance repair garage decided to check..
