I just tried to get into the link and ended up by going into the Home page and typing in Heat pumps in the Search box at the top. ?Then choose the top link. ?Worked fine then.

Here's another link about heat pumps and how they work.

Daikin and Mitsubishi are reckoned to be two of the better ones to go for as far as output and efficiency are concerned. ?I had a link for Daikin heat pumps but it doesn't work any more and, looking at their site, they only talk about air conditioning now, not heat pumps. ?Will have to delve further.

Here's the link for Mitsubishi .

Edited to add Daikin's site, which I have to say, is not very clear. They talk about air conditioning, which doesn't necessarily involve heat, so you have to pick through the blurb to discover they are indeed talking about cooling AND heating.

Oh, and MOST important, don't forget about insulation!!