Ahhaa, I looked at Fig 4 for the honeywell valves and agree with you logic on SPST so thought I had better double check the actuator fitted to the boiler and its different than the spare I acquired (which is Fig 3 and shows SPDT config), it is configured for SPST control (Pic below)
So excellent spot ! thanks
Now back to the fault, could it be the Actuator as it should have permanent 240v on pin 3 and 240v as well on pin 6 when CH demand is on. This is what I have measured on the boiler !
Yeah thanks, back to the 1st item i changed - good lesson learnt to check if the replacement part is the same as the one removed. Will pick up a new actuator tomorrow from the local gas merchant and try tomorrow night.
Yes AGAS going round in circles one could say, but at least I now now how the system works