Electric Heatpumps.

Gideon: 
 

I think that if the eaves don't extend beyond the added cladding (like my Victorian terrace) you get a sloped surface edge. ……

After 20 years it probably dumps a load of rainwater into the wall and causes £10K plus damage.

Perhaps I'm too cynical?

I don't think you are being too cynical. You are, if anything, being optimistic.

Rooves are things that keep the rainwater out. They are built the way they are because that works (and other stuff doesn't). That construction needs to cover anything vertical. It follows that the eaves need to extend beyond the insulation. If they don't, you need to have the roof extended so that they do. My guess (I haven't had it costed because, like you, I wouldn't need it): add another 10%-20% in cost, because there is a lot more eave than there is gable on your typical inverted-V-roof. And don't forget the new gutters. My guess: extension plus gutters will bring you to 20%, and maybe rather more.

Gideon: 
 

I think that if the eaves don't extend beyond the added cladding (like my Victorian terrace) you get a sloped surface edge. ……

After 20 years it probably dumps a load of rainwater into the wall and causes £10K plus damage.

Perhaps I'm too cynical?

I don't think you are being too cynical. You are, if anything, being optimistic.

Rooves are things that keep the rainwater out. They are built the way they are because that works (and other stuff doesn't). That construction needs to cover anything vertical. It follows that the eaves need to extend beyond the insulation. If they don't, you need to have the roof extended so that they do. My guess (I haven't had it costed because, like you, I wouldn't need it): add another 10%-20% in cost, because there is a lot more eave than there is gable on your typical inverted-V-roof. And don't forget the new gutters. My guess: extension plus gutters will bring you to 20%, and maybe rather more.