@FredM67 Yep, I was surprised too, but when it was in the “shoulder months” the over-spec of my heat pump becomes a problem with the default control algorithm.
I also wanted a buffer tank but was talked out of it. I accepted that on complexity grounds. I don’t want to create a system that only two engineers who are on-call can fix and then find out they have 10 other customers to fix first.
I have heard a number of stories of heating engineers turning up on-site and finding an unusual system and not knowing how to fix it or set it up properly. I was hoping that after more than 10 years of heat pumps being installed this would be a thing of the past but it’s still happening now.
However, given what I’m hearing about your system and what I’m seeing with mine I could consider a buffer tank. I have space.
You can see in this experiment that I don’t actually need to circulate all the time. My radiators are serving as a buffer tank because they are over-sized. Admittedly I get more fluctuation in my room heating than you would. Being able to turn all the pumps off is good, it makes our house quieter (yes, I know pumps shouldn’t disturb us, but it’s stupidly quiet here)