So overall at this point, I'm just going to accept that it is what it is.  I've spent hours and hours on this and the bottom line is that the particular combination we have (Evohome + Daikin ASHP) is just not an efficient combination.  If I effectively disable the Evohome system by setting all room temperatures to 25C and control it with the flow valves, it still doesn't help as the ASHP cycles in and out at the super-low flow temperatures needed to keep the house at a sensible temperature.

Thanks for the support and help everyone.

Thanks for the advice for talking to the installer. I will keep this thread updated.

Thank you all!

Ian, the unit has the flow rate for its internal pump to the LLH/CHW loop as well as leaving and return temperatures available from the user interface, which does help a lot.

I'm going to contact the installer to see if I can get them to come and remove the LLH and Grundfos pump.

brilliant. so just double checking, you can you see flow rate and leaving + return temps, both sides of the HEX? if so you've got what you need to tune it once the LLH has been removed.

your installer may have put the LLH in because of concerns about evohome TRV's closing and dropping the flow rate too low. reassure them that you are aiming to run with almost all rads open all the time and that you therefore do not need an LLH and you definitely don't need a bypass valve either. 

Ian, the unit has the flow rate for its internal pump to the LLH/CHW loop as well as leaving and return temperatures available from the user interface, which does help a lot.

I'm going to contact the installer to see if I can get them to come and remove the LLH and Grundfos pump.

Apologies for not drawing out a system diagram, but it's literally what you see in the photo, presumably with a manifold behind the wall to drive multiple circuits to the radiators and towel rails. If there's anything else you need please let me know and I'll track it down.

It's 'presumably' that worries me.  Given how bad what we can see is, who knows what we can't see?  Personally I would presume nothing given the evidence we have!  Not your fault I know so no criticism of you!

also, investigate whether daikin has the capability to report on flow rate and temperatures on the "right hand side" of the indoor units heat exchanger. Other daikin users on this forum may know. If not, fit a heat meter (flow rate and flow/return temp gauge) into the CH side loop. Ideally one that has real time logging. this will be invaluable when you try to balance and optimise the system behaviour between input and output sides of the indoor unit.

I've dropped Daikin a message as I had the ear of their senior heating engineers, asking if removing the LLH and Grundfos pump work be better. They seem completely redundant to me...

Apologies for not drawing out a system diagram, but it's literally what you see in the photo, presumably with a manifold behind the wall to drive multiple circuits to the radiators and towel rails. If there's anything else you need please let me know and I'll track it down.

To suck out energy largely, which it appears to be doing rather well

And to cope with the possibility all your rads are shut down.

I have heard on here and elsewhere of some poor setups, this takes the biscuit.

You will almost certainly be better off bypassing or removing the secondary pump, and bypassing the llh on the flow side so that it becomes a small volumiser in the return.  If you do this you need to ensure at least one rad is guaranteed to be open, or fit a pressure activated bypass.

Then please do what @derek-m suggests to re-baseline how the system operates.

But also please note that this is not professional advice just ideas to be tried at your own risk, as we still don't have a system diagram so there may yet be hidden reasons to do things in a particular way.

When I set the operation mode to be based on Leaving Temperature (difference vs return temperature) then yes, the internal ASHP pump is on, while the Grundfos CHW pump is off.

OMG that's guaranteed lousy performance.

Why do I need the LLH and external CHW pump?

As I have said all along, I don't think you do