OK, my guess. There is a metal clad slow run resistor on the side of the sealed chamber, this may have dry solder joints. More likely it is the boiler flow microswitch(!). The switch (marked FS above r=560 on the circuit diagram fig37 on page 54 of the manual,56/64pdf). When they break (they do break) they can fail showing flow always good - dangerous because the boiler can dry fire, and a nuisance because the fan slow run resistor is never switched in, the fan does not slow run, the pilot is deprived of oxygen and occasionally goes out as stale air builds up. The flow switch is item 27 page 1 figure 1.
It is cheap and easy to get at. An easy job, switch should cost less than a tenner. Caveat - only competent persons should work on this bit of the boiler - mains voltages, and the microswitch is part of the boilers safety mechanism.
Here's the manual:
http://tinyurl.com/y85jwyu
If your fan has sticky bearing it may not be slow running, but it would also make horrible noises.
c
October 2009
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.