Well, it would of course be terribly expensive if I couldn't recoup the money within a few years. In the typical case, heating costs around £2,000 per annum here, and that's brought down to less than £1,000 with ground-source heating. So you take a loan using the house as security and pay it off slowly, and after a number of years your costs are substantially lower AND you emit less CO2. Win-win.

However, if you can't afford the increased instalment payments, it's difficult. And I realise that it's probably less rewarding financially in England.