I've done a bit of reading up on the Money saving expert forums, on the threads about heat pumps.
There are a lot a variables, not least how well insulated the house is. Also whether the radiators are sufficiently sized (you often need bigger ones, in order to get more heat out). The way you use the heating is not always exactly the same as the instant heat of gas central heating.
It also depends on how close the heat pump is to the house (it need not be actually on the wall, and I think is often advised not to be actually on the wall owing to noise and perhaps vibration), but if it is a long way away from the house, then you will lose heat during the transfer.
At the moment, the cost of electricity is very high, and so they are not that cheap to run unless it is a really well insulated property.
I would ask questions about the property's insulation, when the heat pump was installed, which make /model it is, whether the radiators were upgraded, (or if no radiators, what the heating system is e.g. underfloor heating) etc.
There is a key measure of 3 for a well-set-up system. If memory serves right, this is that it generates 3kWh for each 1kWh used - or something like that anyway. If the system score is 3 or more, it's doing OK. If it's less than 3 then it is not very efficient.
