You need to remember that the WC setting controls the target flow temp, and not the actual flow temp. The two will only be the same after a period of stead-state running, provided that the target flow temp can achieved within the constraints imposed by the min and max compressor power that the Arotherm can run at.

In my system (Arotherm+ 7kW), the system is actually unable to run with an actual flow temperature less than about 35C.  At it's minimum compressor power, it consumes about 700W of electricity and outputs about 2.5kW. In order for my radiators to dissipate 2.5 kW of power, the water needs to be at about 35C. It simply cannot run at a steady state any cooler than that.

If the target flow temp (from the WC curve) is only say 30C, then the compressor will run at its minimum power, with the flow temp rising to around 35C. After a while, the ARotherm decides that the 'degree minutes' has reached zero, and it just turns off completely (leaving the water pump running). After a while, the degree-minutes has reduced down to -90, and it turns on the compressor again. The cycling continues. This typical of all heat pumps. The important point here is that the average actual flow temperature will be close to the target flow temp, but averaged over the cycle period. There are plenty of times when the flow temp is well below the target flow temp.

You cannot see, from the SensoComfort or the app, whether the compressor is actually running, or whether it is waiting doing nothing. You can only tell that listening to the outdoor unit or delving into the Live Monitor values on the 'Appliance Interface' (usually by your HW cylinder.).

The other point to make, in response to @majordennisbloodnok, is that the Arotherm+ does nothing whatsoever to control flowrate to achieve any specific flow/return delta-T. It runs the pump at a constant flow rate allowing delta-T to change depending on current power output. If the system is designed to have a delta-T of 5 at design conditions (max power), then most of the time delta-T will be well less than that. Mine is usually about 2.

Don't worry about delta-T. It's largely irrelevant.

Posted by: @robert-m

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When the outside temperature drops further and the lounge thermometer will not rise above 18.5C, I adjust the heat curve to 0.65. When it is 0C or below and there are strong winds I raise it to 0.7. My heat curve has been dropped back to 0.6 since mid-January.

I am in the fortunate position of having independent monitoring (using OEM) of the heat pump and all temperatures.

One thing I noticed quite early on was that the outdoor temperatures, as reported by the Vaillant app and measured with their temp sensor, was often over-reading by a couple of degrees relative to the outdoor temp sensor connected to my raspberry Pi. My Vaillant installer had mounted their temp sensor on the house wall close to the front door, under a porch. Open to the elements yes, north facing yes, but possibly still in a local micro-climate influenced by the house. My OEM temp sensor was only a few feet away, but in a shrub away from the house. In colder weather, these two sensors could differ a couple of degrees, but this difference was much less in milder weather. 

I realised that this would result in flow temperatures that were a little too cold when the weather got cold. 

I moved the Vaillant sensor, mounting it on a fence several meters from the house.

This completely removed the discrepancy in the readings, and my house as remained very stable in temperature since, without needing any fiddling with the WC settings.

@martinrobinson Thanks Martin and apologies for no earlier response as I have only just seen your post.  Maybe I am just getting used to the way this forum work which is very different from another one I use.

Your comments about the position of the external sensor rung lots of bells.  My installer had difficulties finding a spot which complied with Vaillant requirements.  I am going to have to re consider my options.  That said, Vaillant must know of the micro climate problem and yet they stipulate mounting on the building.

incidentally I have managed to mentally park the deltaT figure of 2 and turned it into a positive.  I also now know the pump doesn’t modulate to achieve DT of 5 as I had previously believed.  I am going up a steep learning curve.

Posted by: @davidb

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Your comments about the position of the external sensor rung lots of bells.  My installer had difficulties finding a spot which complied with Vaillant requirements.  I am going to have to re consider my options.  That said, Vaillant must know of the micro climate problem and yet they stipulate mounting on the building.

The micro climate effect is only going to matter if, as in your case, the offset is variable and even then its not going to make a vast difference particularly as you will automatically compensate for the 'average' offset when you adjust the WC curve.  But it definitely would be better to avoid if possible.  Some HPs have the OAT sensor mounted on the casing which of course is then affected by the HP itself, arguably worse.

I cant see how it will do any harm to mount it on a fixed structure away from the building if there happens to be one convenient and the wireless range (if you have the wireless one) is sufficient, or on a North facing wall away from a window so any microclimate effect is minimised.     My installer found a good spot on a projecting bit of the garage roof.