Roger Mills wrote:

Inefficient, yes because there was no boiler interlock. In other words,
the boiler continued to fire just to keep itself warm even when there
was no heating demand and the hot water was hot enough.

Tank and room thermostats are both fitted here, with water and heating
times set separately.

ok, when the heating is "on" and the room stat is calling for heat, then
heat is always going to the hot water tank, up to the limit of the
boiler's own stat.