We have literally just had one installed to replace the oil fired boiler. I can't say yet whether it's worth it or not as although installed (finished Friday) it is not yet set to work (Panasonic themselves come and do that).

Very little disruption to you if your existing installation is up to it. We have grand old cast iron radiators which are supposedly perfect for a heat pump, time will tell on that one. In total we will have 8000€ (IIRC)in subsidies from two different sources for the HP. We are also installing a Poële as well, for which there are also subsidies, and that will be our back up in case of power cuts as well as providing what I like to think of as proper heat and a beautiful focal point.

So yes, there is a large initial outlay, but personally I think that heating that burns a finite resource is not the way ahead.

The simple answer is heat pumps generally use lower temps for your heating which works fine if your well insulated and works even better with underfloor heating or over sized rads.
How old is your gas boiler? Modern condensing gas boilers are very good at being boilers. It might be worth checking if there are grants for a modern gas boiler.

As one who lives in a purely Heat pump house Air/Water allow me to explain, to be efficient your house needs to be very well insulated 400mm in ceiling 150mm in walls and we have 200 in the floor but we have UFH your rads at the moment will be running at about 80C, heat pumps kick out up to 50C so the rule of thumb is that you need a bigger surface area to lift the temp up, so modern and larger rads, all pipe runs well insulated, the external unit does make some noise its a big fan after all pushing a lot of air about, the hole needed in the wall is about fist sized, the internal "Boiler" also make a bit of noise they have electro valves, that open and close, PE is correct in that the SORPS is about 1 to 4 ratio, so for 1Kwt of electricity used you get 4Kwt of energy. Another alternative is a reversable Air con system could be easier to install still need unit outside and units inside, many are available with wall mounted indoor units either high up near the ceiling or down near the floor like a conventional rad, also ceiling flush mounted, but these take more fitting, another advantage is cooling in the summer, this is the type that we have had fitted in the house, in our case mainly for summer use but also as a back up for the UFH we keep the temp at 23C all winter, because of the mass in the floor after been off for 16hrs the temp had only dropped by 3C. You could run all of them off a genny but they are compressors so their start up amperage is high, so probably not economical.

In view of all the info above, for which I thank you, think I'll stick with what I have but put thermostatic valves on one of the 2 radiators. Apparently there should always be one radiator which is permanently on? Plus I'll ask about the age of the boiler next annual check-up. It is very noisy and like Nifty's it fires up every time I turn the hot water tap on. About 1.5 litres of cold water before it heats up. This goes into bottles for the garden so isn't wasted but is a nuisance.