Nigel's the techno guru on these things (and most others).  If it is the thermocouple (easy to identify from Nigel's description) it either needs cleaning (turn everything off and scrape off any gunge) or replacing.  If it's a locally purchased boiler you should be able to get spares at Clibergas (need directions ?).
If you mean that the pilot stays lit but the burners go out, it could be the CO2 sensor.  If so, come back and we can chat about that.