Well, I've given up trying to repair them!
I took the seat base apart and yes - the element is stitched to the leather fabric and not glued to the fabric(apart from the bolster sides). Decided not to have a go at the soldering process as the element is really thin.
Anyways, i bought a mega cheap second hand set and just swapped over the drivers seat and hooray....a hot toasty bum again. Got the same money when i sold the set of seats on, so the fix didn't cost me anything in the end.
I had to replace the seat base as the bolster had gone(foam only,not leather)but i got one from the scrappy(£5) from a passenger side fabric seat, had the same part no's on the seat base.
I carefully peeled the side elements off my base and removed the excess foam from the under side and simply stuck back on to the new base and refitted the leather cover - built the seat back up and its as good as new.
