Saw my mate over the weekend. He says the job is actually quite straight forward.
Remove the seat from the car
Seperate the backrest from the seat base
Remove the seat cover and foam to leave the bare seat frame
replace the heater elements and reasemble
The only difficult bit is replacing the seat cover (especially leather) He says it is a nightmare trying to get it back on exactly right.
He says a DIY job is possible but allow lots of time for getting the covers back on and be prepared for lots of unsuccsessful attempts, frustration, finger injuries, general rage and of course lots of swearing