Two things:
1)It is possible, but unlikely that some debris/scale/dust was dislodged if the heat exchanger fins were cleaned or scraped during the 'service' and some of it may have fallen down the throat of the pilot. Remedy-Clean it out.
2)Thermocouple beginning to fail. It will get progressively worse and eventually prevent you from going onto main burner altogether. Remedy-Buy a new thermocouple. Cost £10-15 depending on your source.
Incidentally, why don't you drag back the person who serviced your boiler in the first place. There is a time test for thermocouple reaction and pilots should always be vacuumed after scraping and descaling. Ask him about it!
cledlyn
November 2008
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