Thanks all for the replies
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.