Now my install has been plumbed correctly I've turned my attention to the buffer, backup heater and external circulation pump as I believe these are put in purely out of laziness/lack of expertise of the designers, whereas in fact they are making the install and run far more complicated and expensive with these items installed.   Get the customers to pay for these extras and pay again in wasted energy using them, so no reason for designers to worry about doing it properly.

There are some key mentions in the Daikin manual which are flags, and just as key there is zero mention of adding an external (CH Circ) pump to the CH system.

the only mention of a field supply pump is here:

My install has a 50 litre buffer, a Grundfos circulation pump and a Backup Heater.  So, according to the specs, with a backup heater the minimum volume of water is just 20 litres, that's about 3 radiators panels/ large towel rails.   I have the equivalent of 11 doubles, which is 144 litres plus the pipework.   So in no circumstance would the install need a buffer tank.   That 50 litre buffer represents about 33% of the total volume of the install, so that's 33% lost efficiency, 33% more expense in bills.

Now I have never seen a design for the system, it will be interesting what the handover pack will include.

I have also done some experiments on the CH Circ pump speed.   

Turning it off or up to full speed doesn't affect the flow rate displayed on the unit, which must mean the internal pump is pushing water faster than the external Grundfos.

Turning the Grundfos off negatively impacts the CH flow, all the Rads went cold even though the F&R temps on the Daikin were at 37/35.  Which means the buffer was acting as a bypass to the heating system without that extra Grundfos inertia

Turning the Grundfos to the slowest speed restored warmth to the Rads and the flow in and out of the buffer was <0.2C (delta).

At the moment my only interest is getting it through RHI.   I'm not going to start messing with the layout as I need that MCS certificate but its something I will deal with later.  A simple bypass of the buffer or ripping it out completely are my thoughts.

Giving this further thought, the buffer tank should only ever be in circuit as part of an automatic bypass system in case of end user error closing all the Rad valves.

......and in my case limited to a 20l buffer tank

some good analysis there. Have you thought of a change in career?  

Seriously though, it isn't right that customers/users end up having to know more about their heating systems that the so-called experts.  Imagine if you had to tell your electrician how to rewire your house.  

Kev