One morning, no heating and the boiler was off. The main power supply neon (on the wall, not the control board) was off and the fuse blown. I replaced the fuse and, with the control board switched off, the orange neon lit so power was back to the board. When I switched on, after a few seconds, there was a load pop and the control board fuse blew. An engineer came, replaced the fuse, and the board then gave error code A05. So he replaced the board - the recommended action for this fault (expensive). All ok for a few days then woke to no heating and the house alarm ringing as the RCD was tripped. Found that switching on the boiler on kept tripping the RCD. I called the engineer again, he came but could not find any faulty connections, etc. and when he turned it on all worked ok (very frustrating).
Anyone had a similar problem or can someone recommend how to diagnose what has been going on?