There are some government schemes around that will give grants or discount on home energy improvements. I'd be wary about cold callers in general, because it's hard to compare prices and they may pile on the pressure - although I know Carillion is a big part of the scheme.

The best option is to take a look at the scheme itself. There will be several providers so you can decide whether you want to use the one who contacted you first.

You can go direct to Warm Front here
https://www.gov.uk/warm-front-scheme/overview

Or to the Warm Homes Discount here
https://www.gov.uk/the-warm-home-discoun...e/overview

There is lots more information here
http://www.homeheathelpline.org.uk/