As someone who owns a Daikin ASHP (with PV no batteries) on a well insulated house, after installation I had to spend a good few months dialing the settings in. Our heating engineer just wasn’t interested in fine tuning the system and impossible to get hold of. So I recommend checking:
1. The controller is setup to a Weather dependent curve this will adjust the flow temp depending on outside temperature. Good time of year to reduce the flow temperature at its lowest external temperature setting(usually -5). Basically keep the thermostat on high (say 30 degrees so it is calling for heat) then gradually reduce the flow temp until the room becomes uncomfortable, then take the temperature setting up to meet a level that suits. Our WD curve was set up to run the flow quite high, if running to radiators then expect to be around 40 degrees or so depending on heat loss. Alternatively if this sounds too technical see whether the controller has the option to adjust the WD curve by simply using the offset function.
2. Check in the settings that the ASHP is heating the hot water, the default on ours was the immersion which added significantly to our running costs – your mum may not use much hot water so possibly only a little saving here.
3. Check the hot water temperature is not set too high, the daikin allows you to run a disinfection cycle once a week which can be timed to mid day when PV generation is at its highest (only applicable to the summer months obvs). You should be able to manually adjust the HW temp via the controller, were on 50 degrees.
It took me ages to get my head around it, im not a heating engineer and the above advice should help get you a bit more efficient.
Additional info: Our WD curve is 38 degrees (-5 outside) to 28 degrees (15 outside), UFH at ground floor and radiators at first floor.