Radiators tepid; boiler shuts down & fires up every 10mi

[ErleMin]

Re: my domestic CH&HW system with Potterton combi-boiler & 2 actuators (CH & HW), Drayton Digistat thermostat in the hall, Drayton TEMPUS SIX controlbox & Drayton HST3 HW tank thermostat. Working well for years. Now CH starts & rads get to tepid temperature - except a very hot one (latent heat?) then boiler shuts off. It fires up again after 10 mins or so & repeats the process. HW OK, if a little hot. So CH no good. Plumber assessment was "CH operative but (SUNVIC SM5203) actuator occasionally sticking". He left CH actuator unbolted & valve open to "give us some heating".
I replaced CH actuator (only, not valve - is this my problem?). No change. Replaced HW actuator (not valve which is a bit stiff), pump & HW tank thermostat. No change.
I notice 2 oddities:- 1) HW actuator doesn't rotate when unbolted from valve when HW is turned off so doesn't seem to be closing HW valve. Actuator is new & operates OK. Does this matter?
2) Turned CH on & both yellow (open valve) & white (close) wires were live. Turned CH off & yellow no longer live (OK) but white still live even after actuator finished turning. HW actuator white wire never seemed to be live so HW valve never seems to close. Red herring?
Welcome any suggestions.

[htg engineer]

Potterton Combi boiler - with hot water tank ?

"HW actuator doesn't rotate when unbolted from valve when HW is turned off so doesn't seem to be closing HW valve. Actuator is new & operates OK. Does this matter?"

The actuator is new and operates ok ? but doesn't turn ? it's not operating ok, if it's not turning.

My advice is let heating engineer sort it, or get another heating engineer to take a look. Can't really help if you don't know what type of system you have.