Good luck!
We have an old GEC Nightstor electric boiler running wet radiators. It is brilliant, but we also get through 16MW electricity per year, that is everything, heat, domestic, workshop. It seemed sensible to look,again, at a heat pump.
After spending a long time searching, there are many websites to look at, from the UK posting feedback and comments ASHP is not a great way to go. This means the total cost not only of running, but also buying and maintaining it.
Doing the sums I came to the conclusion that I could save more money by running a diesel generator into the Nightstor. It seems that diesel is available for £0.70 per litre, so £0.075 per kWh. Diesel is 11.6 kWh per kg, sg of 0.8. The genset is about 60-70% efficient but the losses are heat, so you put it in a shed and plumb the radiator into the house. If the shed is insulated then it to will be warm, like my workshop! It would be cheaper if you can use heating oil in the engine, but savings might be more illusatory than real.
The one unknown in these ASHP calculations is how much they cost in maintenance. The Nightstor costs nothing per year in maintenance, unlike a gas or oil boiler. I once cansidered a log gassification boiler, we have a wood so the fuel is free, other than the cost of felling, shifting and cutting up, which is significant. The cost of buying the gassification plant was horrendous, can't really remember, £10k? And then you have the chimney sweeping etc etc. Simply didn't add up.