DC voltage going to boiler

I have a Boilermate A Class OV, I got a service engineer out to carry out a yearly inspection on my Ideal boiler and when he checked the incoming voltage to the boiler he found that there was 240 volt DC going to the boiler, he said there was nothing else that he could do until this was rectified as it should be an AC supply to the boiler. I found this a bit strange as i am an electrician myself, so i tested the supply and found he was correct. I contacted the Gledhill technical support and explained the problem to a technician by phone, we tried various things which included checking that the incoming voltage to J5 on the GT490 PCB, this was 240 volts AC, I removed all other plugs from the PCB leaving only the plug for J5 connected and measured the voltage on the PCB outgoing pins at J3 and still found that I had 240 volts DC on the PCB pins. The Gledhill technician said that he had never seen this problem before and it must be the GT490 PCB that was faulty. I bought a new PCB and fitted it, but i still have the problem, i contacted the Gledhill technician and said it still had the same fault which left him completed stumped, he also contacted other Gledhill technicians and they had never heard of this before. Could anyone provide me with an answer to this problem as it has also left me stumped as well
Col1967

Hi,
How did you get on?

Hard with out being in front of the unit.

But I would have checked time clock / disconnected it.

And relay.

And back up heater.

Mike