Glowworm Ultimate Boiler - Electrical Problems?

I have a Glowworm Ultimate Boiler, I've had it for around 12 years and just recently it stopped working. The boiler is in near constant use. I have a service contract with Scottish Gas, recently it just stopped working. Seemed to coincide with the bad weather we were having (may be a coincidence)

When the engineer came out he read through all the diagnostics on his PC, he said everything looked ok but it was quite possibly water that got in that had tripped the internal fuse (up near the fan). He replaced the fuse and hey presto everything was fine .... for a couple of hours .... bang it tripped again.

Call to engineer, they'll be out next morning ... afternoon arrives and then engineer has a look after me explaining what had happened. He replaces the fuse on the main circuit board and says it was probably water in there that was causing the problem so drys it out with a hair dryer and switches on. Fine .... for 3-4 hours and then .... bang it trips again.

Now to me (and I'm not technical about these things at all), this sounds like some wiring or electrical issue. I popped open the circuit board to look for any water and couldn't see or feel any! To me it's as if something is heating up and expanding and at that point is touching something else hence the 'bang' of the fuse on each occassion.

Any thoughts or suggestions?