Just has someone round, Turns out the expansion vessel has split. So thanks guys, when i found that out i was already aware of what component it was.
Still not sure why it won't power up, pcb board i expect. The fuse on the plug to the boiler is all good.
The engineer did say even if we replaced these things there is a chance it still might have problems.
From peoples experience when a problem like this pops up is it better to get a new boiler? Or cough up the costs of repairing it.
Might be worth contacting Vaillant for a fixed price repair if you feel you have tried all applicable fuses and you think it is the PCB.
If the expansion vessel has split you can have a generic one fitted outside the boiler and the existing one's connection capped which wouldn't be that expensive.
Of course if the PCB has gone then fixed price repair would be way to go but make sure they will fix multiple faults in the price.
That way you will get the EV and the PCB if it is dead done too.