@optimistic-optimiser I have has a Daikin for a few years and done a lot of checks/observations.

so 

So on this question of compressor longevity, I have a Daikin Altherma 3 11kw installed by Octopus and I found that when the weather was a bit warmer the other day, but cold over night, that the blue light on the MMI kept going out and the power consumption of the heat pump dropped from c.1kwh to a few hundred w/h. 

**all of below is my logic with lots of reading over last few years **

If the ASHP feels the return water is high then it will go from 'blue light' heating to 'no blue light# but water flowing into the pipes.  the very same as when it you set the water to heat in the middle of a room schedule.  have you noticed that the flow water is very hot ( its just finished a hot tank cycle) but it will now switch to heating the home again, the rads get hot BUT the blue light isn't on. = its using all the excess from pipe/buffer until it get to a low temp vector then starts the compressor to start bring the in/out flows to correct temps.

it will also do this "no blue light" water into the pipe is the outside temperature is ~<5 degrees., this is frost protect, I turn my off but is still overrides and keeps enabling it, as now in April 2026, it sunny and will not snow but the HP can't see the weather just temp so trying to protect the system from frost.

the modulation was at 5 degrees and the overshoot was set at 2 degrees.  modulation is good if you want to slow the system down to think before it want to cycle down - (I force mine to use fixed temp with -2 degrees room thermostate offset so my system never reaches temp and I increase my flow temp for overnight TOU rate, but this is off topic)

I keep my overshoot to 1 degree as I don't like the HP to increase its range if the temp outside drops , as I don't want the extra pull of power for very little gain.  - I control my flow temps manually, cop, scop isn't a target for me.

Their answer was to recommend increasing the overshoot to 4 degrees.   that from training they had with Dailkin, I had 6 engineers in my home during my install as it was so new for Octopus.

Am I right in thinking that the compressor has shut down when the blue light on the Madoka goes out and this is a bad thing?  Yes and no. - Yes as your paying for it from the grid, no as the compressor isn't working. which is a lesser evil - you decide.

If the heat pump knows what its target temperature is and it has scope with the modulation and overshoot settings to adjust, why isn't it doing so?  from my experience its logic is to meet middle ground, having it on say fancoil and it reacts too fast, all of this you can see/test if you do not use weather compensation , if you play with fixed you and really see what occurs to HP when your 'leaving water temperature is at 40 degrees' then you change it to 36. the system going panic mode - it increased the litres/m from say 7 to 20 to get the temp down to 36 as fast as possible  (like someone has a gun to the HP head) , & is so dumb it turns off compressor as it over cooled itself , system cools down to say 26 degrees. flow rate goes to 0  & then is uses ~1.5kw electric to bring the temp up to 36 degrees, then the litres per min at a high rate until about 20 mins, it all settles back down.~~AKA cycling 

again I have spent a lot time with mine, hard to jot down this in a post as its about 1 years worth of experimentation and now I don't change anything or care about performance, COP.  it just runs my house at 23 degrees and I am happy.