right... so the good news is it's possible to fix this problem.
It's not the motor but the "gearbox" mechanism that it's attched to. well.. in my case anyway. I bought this
eBay - The UK's Online Marketplace although I should point out that ETKA lits the seat base as without motor. whether the part bought from the dealer will have the gearbox bit I don't know. I'll double check the old part in case there's a separate part number on it but I don't think so. seat base is part number 8E0 881 158.
I removed my seat and laid it on its back in my garage to get access from below. I could have taken the 24mm nut and two torx head bolts out in that position but the inner side of the 24mm nut is a thick torx headed bolt and there isn't enough room to slide it out unless your seat has broken while at its highest.
If it's just your motor that's broken then you can detach it from the gearbox by undoing the two very small trox headed bolts you can see by looking down the length of the motor. To get the motor out from the seat base you can unclip the electric connectors just inside the plastic trim (behind teh switches). that will give you enough space to slide it out and the new one in again. remember to unclip the electrical connector by cutting away the tape and then push the connectortowards the back of the seat.
So anyway, to get proper access I had to undo two torx bolts holding the horizontal bar that has a motor mounted on it for operating the forward and back movement of the seat along the runners. I also had to undo the large bolt that forms the pivot for the front part of the up and down lift mechanism. see these pictures for what I mean
the runner at that side can now be gently pulled away a bit, which will bring the gearbox part away from the underside of the seat so you can slide the thick bolt out. This will leave you free to lift out the gearbox and the metal plate it's attached to. When i did this I accidentally unclipped the rod that operates the forward-backwards runner but it clips back again into the motor at one end and the mechanism at the other end.
Get the new gearbox thingy in place, put the thick bolt back through and tighten on the 24mm nut at the other side. Now get the three bolts back in again that allowed you to drop the runner down. that took me a bit of jiggling to get things lined up.
It may be possible to put all this back in place with the new motor still attached to the gearbox but I had the motor off already so I did this and then slid the new motor in, inserted the two small torx bolts holding it onto the gearbox and put the electrical connector back in place.
It's worth mentioning that if/when you disconnect the motor from the gearbox there's a piece that fits into the geabox in the form of a metal "screw" (the thread of the screw turns against the teeth of the mechanism to create the movement). The other end is a flexible bar that inserts into the motor. I removed mine while working out what went wrong with the old one and realised that the end of the screw needs to be engaged in the mech before the motor is attched. if you take it out then make sure to put it back in, turn by hand clockwise until the screw appears to be physically moving down into the gearbox and adjust back and forward until the plastic edge is flush to the end. that will make sense if you find yourself doing this. otherwise.. probably not. the screw wouldn't catch in the broken gearbox, suggesting to me that the problem lies within it.
new gearbox and motor connected, seat back into car and Robert's your mother's sibling... all should be well again. It was for me.
