@M.isterW Thanks - I am not averse to changing the settings, and I have found a useful youtube video which looks at how to change them. However, I have found that there is a lot of variation in what people think is the best way to set them up, and I am trying to see how much it costs to run at the moment with the settings the installers used. Our winters here are much colder than elsewhere in the UK. We are on the same latitude as Kamchatka and Talinn and last winter we had temperatures of -16 on at least one occasion. I expect the running costs to increase substantially, One of the things that will add to that is the defrost mode. But another factor that may increase costs is changing the water settings. I don’t need all the hot water it produces at the moment, but I wonder if it is a false economy to put the hot water on a timed setting (which my neighbour has done) as this will mean in winter it is starting from a low base every time it needs to heat the hot water, whereas if the hot water is automatically heated it will just be a top up. The cylinder is located in the coal shed at the back of the house, which is dry and surprisingly warm in winter but it is immediatley next to the cold weather so this may lead to the water cooling down a lot between timed settings. Also,  I need to build a ‘canopy’ that will protect the pump from the snow falling off the roof (which it does…) and from the two foot icicles that form on the gutter….so it is a work in progress and changing the settings is just one part of it. The heat pump is located just to the left of the window and bins in this photo…I am anticipating  lot of digging!

I changed our water to timed, twice a day. In the winter it was a pain having the heating switch off at critical times when it decided to heat the water. We also have cheaper electricity at certain times of day, so that's when we heat the water cylinder.

I also adjusted the legionnaires cycle to run during the cheap electricity period, although I'm considering switching it off completely. There's some good information on the Heat Geek website that explains why it isn't necessary.

@M.isterW My problem with setting the water on a timer is this: the water is what heats the radiators as well as being your hot water for use for washing etc. If you set the hot water to twice a day, then it will cool  between those times and the pump will have to work harder to reheat it first in order to heat your house, thus cancelling out the savings you might make by only having it come on twice a day. And in my case, the temperatures at which the pump will have to operate in winter will affect its efficiency anyway, especially round the 0 mark in particular, which is where Mitsubishi admit that their pumps struggle.  So although there will be times when the pump needs to prioritize water over heating, the time taken to heat the water will surely be shorter than if it hasn’t been heated for several hours with an outside temperature below 0? I do not have solar panels and the battery that could supplement my electricity, I couldn’t afford them.  So my priority is to keep the ASHP from having to work too hard too often. I will definitely look into setting the legionella process so that it happens in my off peak hours (I have Economy 10), thanks for that advice. 

I’ve switched the legionella cycle off. I ran it recently to confirm the immersion worked ok but that’s all. Your heating circuit will be separate to the hot water circuit, there should be a valve that turns from one to the other. The system sends heat to either the heating or the hot water cylinder depending on the position of that valve (which has an electric motor on it). This means that when the hot water is heated the radiators are not. I run my hot water at 3pm when the radiators cooling is least annoying, and at 3am when I’m in bed !

Fab snow btw