mts200b

Hi - I have inherited an old ST6400C when we moved in 7 years ago. I'd guess its 15 years old looking at the agr of the boiler etc. Generally been working OK but I recently replaced the mechanical dial wired thermostat ( see pic ) with a meross mts200b ( see pic ). Since then ( it may only be...

I am replacing my old Drayton RTS1 thermostat with the Meross MTS200B and I don't get the heating switching on. I thought it was going to be straightforward but at this point better to ask for help than screwing something. The old thermostat wiring is this one: The new thermostat has this...