A friend had one installed a few years ago, he did though retain his gas boiler.. he said they coped generally, but in really cold spells they had to also use the boiler.
The newer systems (I believe) can be programmed on the smart controller to recognise when it’s cheaper to being the boiler on, I don’t think they both work concurrently, I’m sure there’s maybe a few on this thread who could answer that though, a think we’re going to see a big push over the coming years to try and get more installed, my kinda issue with it is there appears to be a lot of upheaval when retrofitting, pipe size increase, rad size increase etc......but if there’s funding there then there will be a big uptake I reckon
The problem with any government funding of this type is that it just distorts the market. Sharks will see a chance to make a killing and prices will magically go up by the amount of funding available.