HE150 - Blown fuses and RCD trip

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?