John Rumm used his keyboard to write :
I have ordered up a pipe stat, to fit on the flow. Intention is to set
that to 60C wired in series with the room stat, to limit the call for
the boiler to a 60C flow, so it stays condensing most of the time. I

Won't that just make for more cycling?

That is a possibility - but it depends on the hysteresis of the pipe
stat. Its a bimetal type, but Danfoss don't spec. the hysteris, only
the setable range. I'm hoping it will cycle no more than it would
anyway, operating only on the room stat.

Its a Danfoss 041E0010.