Nest Thermostat wont stop calling for heat (Y plan)

[robdupre]

@stem and @Johnmdc

Thanks for the responses all sorted and very simple issue.

For completeness, the red wire that runs to the boiler's pump live was incorrectly wired with the boilers switched live and as a result was keeping the boiler on indefinitely. Thermostat calls for heat, boiler comes on, boiler pump wire goes live (as boiler wants the pump on), thermostat stops calling for heat, but as my boiler must have pump overrun, the we are stuck in a loop where the pump overun is keeping the boiler on which in turn keeps the pump overun on.

Stupid issue, I misunderstood the role of the boiler pump wire, I thought it was a internal boiler pump (as i had with my old houses combi boiler)

Thanks again everyone, great forum much appreciated.