Hi

If my experince is anything to go by forget air source heat pumps if the outside temperature is likely to fall below 5 degrees Celcius . You will have to run pump 24/7 and if you are lucky you may get 40 degress plus hot water .My system is made by Heatking ,12kw rated output,it is noisy and struggles to keep the underfloor heating going ,it is useless for the first floor fan assisted heaters (more like coolers ). I am now having to consider an electric boiler to boost the water temp for the radiators and give me domestic hot water.Economy 10 is worth considering.If you take the installation costs along with the high energy usage this is not a solution I would recommend .Electricity prices are a rip off,as are most of the utilities in the UK.