All of which is very nice, but does not really addres the original post ( nowt knew there then ! :neenaw:
Richard, it would HAVE to be electrical I think mate...
two reasons spring to mind.
1) The radiator capacity on the Bird is fairly critical, and if increased the engine will not get anywhere near working temperature ( witness the fact that unless stuck in traffic, or in very warm weather, most Birds rarely get past the cooll part of the temp guage ! )
2) Removal of the seat might become a bit of a pain if it has a couple of pipes attached to it... although I suppose you could use a variation of the Suzy's used on artic's.. You would also have to be pretty careful about filling the system, as the seat is at a higher point ( I think ) than the top of the rad.. So you would have to either fill the system with the seat off and dangling down, or supply some method of bleeding the air from it...
All in all, proly be somewhat easier to just introduce a heating element under a noce gel pad Richard.. and I am willing to bet that Melboy would be able to do the work required at a not too unreasonable cost :-: