Re: Lets do the maths
"Then you have all the stuff from the 50s and earlier without any form of insulation or cavity walls. Pretty much there the only option is to start again so you are back to square one."
I don't know about that. My 1920s home is actually very well insulated and requires very little cooling help in Summer - especially with a large-ish tree out front shading us much of the day.
The steam heat through radiators is surprisingly efficient (and comfortable) and we've had several tradesmen warn us against getting anything new to replace it (barring a total breakdown or other disaster) as nothing around is nearly as good. I don't know that they're correct, but I do like my utility bills being as low as they are (relatively speaking, of course).
At our latitude, solar is only an option for Summer bonus energy.