I've often seen this wiring diagram for Y plan but is the cylinder stat wired correctly?, should the cylinderstat terminal1 go to the boiler+circpump, terminal2 to the MPV grey wire and terminalC to (from) the programmer "HW OFF"?.
The below wiring suggests to me that with "HW Off" and with the cylinder stat satisfied that there is still power to the boiler+circpump.
The below wiring suggests to me that with "HW Off" and with the cylinder stat satisfied that there is still power to the boiler+circpump.