Probably airlocked. Or worse still airlocked with an auto by pass on the system. Turn all rads off except one and see if that heats up.
Make sure system is filled and pressurised. And maybe u boiler is firing for pre heat hit water. I think they have a dhw store on them fs combis
Lol. You'll be sorry.
A. U can't change just the head u gotta do the whole lot.
B. Probably gonna need a water pressure sensor. That's mainly the fault f75. You know when u drain it and pressure still reads 0.4mbar and the boilers empty.
7 years ago or not this customer sounds like a right pain. I'd have left more than the old nuts behind if someone was hovering round me. Plus the next engineer can see its had a g10 seal with the old-style nuts left