I suppose "No external control" could mean "no PWM"

Yes it does, I've confirmed this on my own unit when I changed the pump speed. Due to the low loss header on my pre-plumbed unit and 6KW outdoor unit, I've gone for a low flow speed to the outside unit, and high speed to the radiators, which has improved the COP somewhat.

At the time that I lifted off the PCB to get to the pump, I was on setting 5 and I haven't checked at lower settings because I need everything I can get from the pump! I suppose "No external control" could mean "no PWM"

When I was using Havenwise I was using the Mitsu PAR-WT 60 wireless controller and only using a number of Shelly Gen 3 devices to check on individual room temperatures and they are all using battery power.

However, I have abandoned Havenwise and 'adaptive' and reverted  to weather compensation. Your explanation is probably one reason why Havenwise just didn't seem to be capable of working for me, although the layout of the house is also an important factor.

Havenwise did ask if I would use a Gen 3 for control, but I think even they were not confident that it was a solution. 

Steady green means that the pump is running full speed. If you slow the pump down, then it'll start flashing due to the PWM signal. Speed 5 is the top speed and no PWM.

On the Havenwise and the Shelly thermostat, do you run the thermostat on battery or USB power? I had problems running it on battery as the updates to the temperature were way too slow like every half an hour max with Havenwise adjusting the flow way too quickly as it was upping the flow temp and not seeing an effect from that, when on USB the updates are much quicker, and so it sees the effect much faster thus able to get the right flow temperature.

but it will probably be worthwhile to box it in with additional insulation in view of the higher temperatures.

Once its all working then maybe yes.  Hopefully the re-pipe will reduce the flow temperatures needed!

In that case, that is the first thing I will try.

This tank is located in an attached pump room and insulated, but it will probably be worthwhile to box it in with additional insulation in view of the higher temperatures.

Obviously, in the case of a buffer, it is essential to make connections across the tank, but is it just as desirable in a volumiser? 

No, what you want in a volumiser is to ensure that the whole tank is 'engaged' ie you dont get dead spots or stratification, otherwise you are reducing its effective volume.  So (at least as far as I can see applying 'first principles') in and out on the same side is fine so long as one is at the top and the other at the bottom (preferably in at the top and out at the bottom so the flow is in the opposite direction to any stratification which does occur).

One of the most effective changes that I could make, is to reconnect the 3 way valve to the top left connection on the volumiser, (where it was originally in buffer tank configuration) and this would eliminate a number of reducers and elbows and a length of pipework.

The bottom connection on the same side of the tank would give the straightest, simplest route for onwards to the radiators. 

Obviously, in the case of a buffer, it is essential to make connections across the tank, but is it just as desirable in a volumiser? 

My suggestion of putting the volumiser in the flow rather than the return was more aimed at reducing the number of elbows rather than an inherent advantage in one or the other.

Fair enough

I have also been wondering about the 3 way valve and whilst I think it is generally a reasonable choice, I do now wonder about the sizing at 20mm

Thats 1m head loss at 32l/min according to the data sheet.  Based on the performance curve you need somehow to save about 3m of head loss; swapping this for a 32mm one wont be cheap, but will make a big contribution.

It may well be necessary to fit a second pump, but I don't want to go to the expense of a new pump before seeing what can be achieved with improved pipework. I don't favour installing the old pump because, although it is a PWM, I don't have the speed change control.   

I think thats sensible TBH if you are prepared to experiment.  Its flow rate that matters and the heat pump measures that, so it doesn't have to be done in the heating season.  A good plan for route and pipe sizing (as well as some approx calculations) is the key to making this work, and you perhaps consider some temporary 'experimental' links to diagnose head loss in each part as you go along so you know where to focus.  I note that the rating plarte for the IDU says 'allowance for 3 pumps' so Im guessing that there are additional PWM connections available if you need them.

Im pretty confident that the problem is now identified, which is more than half the battle!

Dealing with new questions first:

My Ecodan spec is an EHSC VM20.UK

I had previously obtained the pump curve as published by Mitsu for the EHSC, but once I gained access to the Grundfos pump I felt that this would be a much more reliable source for information on the specification. here:

Note that this model has neither on board control or local status indication and in this respect, resembles the one in the brochure referred to as UPM3 with two PWM profiles. Consequently I am reasonably confident that this is the data we can use.

I have also been wondering about the 3 way valve and whilst I think it is generally a reasonable choice, I do now wonder about the sizing at 20mm

See below:

However, the more I look at the pipework, the more I am dismayed at the number of severe elbows, e.g. this 3/4" one after the 3 way valve.

My suggestion of putting the volumiser in the flow rather than the return was more aimed at reducing the number of elbows rather than an inherent advantage in one or the other.

It may well be necessary to fit a second pump, but I don't want to go to the expense of a new pump before seeing what can be achieved with improved pipework. I don't favour installing the old pump because, although it is a PWM, I don't have the speed change control.   

I continue to search for Pex pipe and fittings and so far I can only find straight couplings or severe elbows in 22mm and there is little available in 32mm.

Incidentally, I note that the current return from the volumiser is, in fact, 32mm, so I would aim to retain a longer length of this if I do re-pipe.

I am still trying to think of someone I could trust and that would be willing to do the re-piping, but I will almost certainly wait until the end of the heating season now.