I had continuing problems with a weak pilot flame on a completely different boiler - the weak flame was not providing enough heat to properly convince the control board that the pilot flame was alight. The thermocouple signal to the control was too small because of the weak flame. Because the system was unable to "see" the pilot flame it shut off the gas as a safety measure....
The cause was dirt blocking the hole in the pilot flame injector. After cleaning it the flame was twice the size and everything worked a treat. Private persons are not supposed to interrupt the "gas-way" - a certificated engineer should make safety checks after this kind of work.
Peccavi
October 2009