I removed old sports seat - 2 nuts at front at 2 at rear - as long as power to seats its easy to move them to and for to get access to the bolts/nuts.
Slide the black cover off the wiring connection and remove bolt anchoring seat belt. Once free seat will come out - but its heavy!
I installed the passenger seat, connected up the existing wiring and hey presto passenger heated front seat worked.
So the heated seat switch will only come on if seat circuit present. The relays and fuses must be all present (car is 2004 3.0 X5 Sport)
Drivers side bit more complicated - unable to move the donor seat to and fro in the breakers yard - so undone the 4 nuts holding the seat to the 2 rails.
Took out drivers seat from my car and swapped over the rails from my sports seats - to the donor seat - fairly straightforwards.
Installed the seat and all working, again heated switch switch now active.
Only problem - is when I installed drivers seat, there is now an airbag light on dash 0 so I guess there is a connection problem or switch (?) faulty from donor car.
Anyone have a circuit diagram to try and track down the fault ?