Here's an update.
I bought the e9x heated seat pads from the lad on eBay in Gar's link above after he confirmed they'd work in an e70.
It turned into a mini group buy as Andy and Conor got in on the act so we got a bit of a deal.

I also found the heated seat button I need, on an e70 these are incorporated into the air com controls so these are on the way now too.

My plan is that when it all arrives I'll get the buttons fitted, get any wiring and coding done and test fit the heat pads but not under the leather. If it all works 100% then I'll get someone to carefully remove the leather, fit the heat pads and replace. Possibly Nolan's but we'll see.

My next move is to call Mick in Germaniks tomorrow morning.

Thanks to Gar for getting the ball rolling on this.