Neither flowrate or DT are that critical, provided that both are within a range that allows the transfer of sufficient thermal energy under all expected operating conditions.

I would suggest that the water pump speed should be set to provide a DT of approximately 5C,when the heating demand is in the 50% to 75% of overall maximum. Don't be concerned if the DT varies from 5C, provided that your home achieves the desired indoor temperatures.

However, I still do not see how knowing the flow in litres/ minute helps me to evaluate the best pump speed setting. Am I correct in saying that it it is Delta T that counts and if so, what figure am I aiming for? 

As I understand the Ecodan Hydrobox, it has a single pump with 5 speeds available and I don't see anything which indicates that it compensates in any way, after a speed is selected. The resultant flow must, surely, depend on the pump curve at the chosen speed and the head imposed by the radiator system.

I have no means of measuring flow and I suggest that it is not a meaningful parameter. At the moment, I don't have temperature measuring on the radiator flow and return, but I can arrange for it to be fitted if I go ahead with the modifications involving removal of the secondary pump. 

Am I correct in thinking, therefore, that the optimum pump speed setting must be the based on flow vs return temperatures and that a reasonable aiming point would be 5C.?

What I've seen with out Ecodan is that it doesn’t need a variable pump with PWM control.  The system selects one of the 4 part-load outputs then modulates within its range. This makes it very versatile in its output range. And provided with only one fixed flow rate.

the below chart shows the new optimum flow rates.

The new data book has also renamed the 4 compressor outputs see below

I also know the heat pump, when fitted with Mitsubishi cylinder they are usually fitted with plate heat exchanger and an integrated (adjustable) circulation pump. But when they have a 3rd party cylinder the circulation pump usually needs to be adjusted manually.

I have the contractor visiting who is primarily a solar installer but he added the DHW system and I have a lot of respect for him, although he has some fixed ideas.

I have started to explain the perceived benefits of taking the buffer tank out of circulation, but it occurred to me that I am uncertain how I will decide on the speed of the built in Ecodan pump.

What method will I use to evaluate the performance at the available speeds? 

I have just remembered that with Ecodan's the water pump is external to the heat pump, and the water pump speed may not actually be controlled by the controller, it will just be started and stopped. If this is the case then the speed will have to be set on the pump itself, as James suggest with reference to the flowrate reading on the controller.

If you have two cables connected to the water pump, power and control, then it should be possible to vary the speed using the heat pump controller.

Thanks JamesPa - have only just picked this up.

I have the contractor visiting who is primarily a solar installer but he added the DHW system and I have a lot of respect for him, although he has some fixed ideas.

I have started to explain the perceived benefits of taking the buffer tank out of circulation, but it occurred to me that I am uncertain how I will decide on the speed of the built in Ecodan pump.

What method will I use to evaluate the performance at the available speeds? 

I agree with James. I think that I am correct in saying that the water pump speed on the Ecodan can be varied from 1 to 5, so probably start with the pump set at 3 and see what DT you get. You may find that you need the pump speed lower in Summer and higher in Winter.

There is a recommended flow rate range for each model (its quite a large range), and I think you might be able to display measured flow rate.  if you can, set it in the middle to start with.

I have the contractor visiting who is primarily a solar installer but he added the DHW system and I have a lot of respect for him, although he has some fixed ideas.

I have started to explain the perceived benefits of taking the buffer tank out of circulation, but it occurred to me that I am uncertain how I will decide on the speed of the built in Ecodan pump.

What method will I use to evaluate the performance at the available speeds? 

I agree with James. I think that I am correct in saying that the water pump speed on the Ecodan can be varied from 1 to 5, so probably start with the pump set at 3 and see what DT you get. You may find that you need the pump speed lower in Summer and higher in Winter.

Otherwise you will have to look in the installation manual unless someone here is familiar with the specifics of ecodan.  Or just set it at the middle speed, and see if you get a DT flow-return of roughly 5C at the design OAT (or in proportion to Room temp - outside temp at higher OAts unless the heat pump modulates the water pump speed), with all emitter valves open.  Its not particularly critical ASFAIK so long as its sufficient.