Hi there, Ade,
May I offer a few observations regarding heat pumps. Please forgive me if I'm 'teaching Grannie to suck eggs'!
The figure of merit for a heat pump is called the 'Coefficient of Performance' ('CoP'). CoP equates to the ratio of the heat you move divided by the energy your pump needs to achieve the moving, so big is better. It's the absolute temperature (i.e. degrees Kelvin) at the heat source divided by the difference between the source and output temperatures. (It might be the output temperature rather than the source temperature, I forget. It doesn't alter what I'm going to say next because, with absolute zero being -273° C, they're almost the same thing!)
The point is, to achieve a high CoP you need to keep the difference between source and output temperatures small.
So, if you're using the heat pump for heating, you need to prevent/avoid the heat gathering system from icing up. Ice is a very poor conductor of heat so the pump has to lower the temperature of the gathering system to 'suck' the heat in through the ice. The temperature that is actually effective in the CoP calculation is the temperature at the coldest point, if that falls, the operating differential gets correspondingly bigger, reducing the CoP.
In my view, that means steer clear of air source systems and give preference to ground-water or running water sources. Put enough tubing in the heat gathering system to avoid the source water freezing. Can you chuck a couple of hundred feet or so of copper pipe into the Mersey?