Thanks for help so far Ian. Just to clarify the engineer took the transformer and tested it. He doesn’t carry test rigs with him in the van.
Are there any other qualified gas safe engineers that have came across this problem that could advise ?
Switched off the supply, unplugged the transformer, had test leads on the input and checked the supply on output of the 4 pins, each pair was giving the correct output
The fuses have been checked and are ok so that rules them out. There is 240v at the boiler connection so the 3 amp fuse at the spur has to be ok.
The fuses on the PCB board were checked with a multi meter and all showed ok.
The plug in transformer on the PCB board has been tested remotely and...