Thanks for the reply but the issue is now sorted. It was a faulty zone
valve on the boiler that failed after a power cut. Nest was was talking
to boiler and all okay. Only got sorted today however ❄️略
Do you mean the zone valves, they're not electromagnetic I don't think.
Just motorised and go from Auto to Manual Open I have just notice the
wire that seems to have snapped off from the block though (circled) I'm
assuming that needs to go back in???...