it struggled with this last year. We were coming down to 19 degrees typicallly.

Since the external heated Volumiser has been added it’s been a big difference so far. Comfort temps being met easily. Whole house at 22 and no longer calling for heat when I get up. 

It’s set for 21.5 so will go up to 22 before the heating demand cuts off.

house was lovely and warm this morning. All comfort temps comfortably maintained. 

one other thought I have ahead of winter is my Delta T. This is currently 6 but I wonder if I can get it to 8. I have the pump speed on the medium setting. If I lower that would I likely get an increment on the delta T? 

it has underfloor all downstairs. 
8 rads upstairs.
3 additional rads upstairs in the garage. 
on the non visible side of the garage roof it has 5.46kw solar array and contained inside the garage is a 10KW battery. 

I would like to add more solar and another 10KW of battery storage. Maybe a project for next spring. 

house itself is 294sqm. Year 1 all good. There had been a debate about whether to install 13kW or 17kW model but with the house being quite efficient and underfloor heating downstairs throughout, the installers plumped for 13KW.

Last year we built a large garage with office/man cave above. Downstairs is 72sqm is currently not heated, although the plumbing is there. The upstairs is  48 sqm and it is heated. 

After adding this, the system didn’t deliver comfort temps quite as well. Grant believed the system needed more volume and recommended the external volumiser with immerser. On the original install we had no practicable space for a LLH.

total energy bills for a 4-bed all-electric house at under £1K at year,

That sounds cheap - well done. Much cheaper than us.

The house is 8 years old and well insulated.

I have been generally following the thread with interest.

Couple of things I am having trouble to reconcile 

House only 8 years old and well insulated as well as requiring a big heat pump, with an additional heat source.

Is the house just very big?