There is some play in the base of my drivers Sports seat, you can rock backwards and forwards a bit, feels like you are pivoting around where your arse is
check if its bolted down? if it is (of course) then pull the seat out the car. take the back off, and check the fitting for the backs, it could be the screw mount has snapped
I really hope its not something as simple as that in a way, but at the same time hope the solution is as easy
Its the whole seat that moves, base and back together. Thinking about it on the Sport seats there is a lever that controls the angle you sit at, its the same kinda movement, id better check that out
Had a look under them today, they are bolted down firmly, the movement seems to be in the linkage that sits at an angle connects the base of the sear frame to the part that bolts to the floor.