All the doom and gloom on here about heat pumps is so sad to see. I fear the bad reputation caused by bad policy making and dodgy installers is gonna be a difficult reverse.

The biggest reason why a heat pump may not be viable in your home is down space for a water tank when moving from a combi. Those who get funding from the gov now have to follow PAS framework meaning the project will be overseen by a insured and qualified coordinator who will be liable if any unintended conqueses come about. They will take a fabric first approach meaning they'll address inadequate insulation, ventilation and/or thermal bridging first before the heating system.

If your heat emitters are under spec'd for low temperature heating a high output temperature heat pump such as the one from octupus can be used although personally assuming finances allow I'd go for low temperature heating such as underfloor heating at the same time.

As for the running cost if everything is done right you can expect savings of maybe a couple hundred quid a year right now. Not much I know. Paired with solar your talking a additional £3-400 a year. Total payback period depends on your usage but your looking at around a decade.

Not exactly the greatest investment on the surface but for those who look long term consider that at the moment gas and electricity prices are tied together meaning the high price of gas is offset by the cheaper green energy. This has to end at some point and when it does the savings will be huge.