As a follow-up to the Dead Boiler post here's what I've worked out probably caused the problem.
As well as talking about it here I posted to the uk.d-i-y usenet group about it. One response talked about a boiler's fan going when the wall near the flue outlet was rendered and rubble made its way down the flue to the boiler. That hasn't happened to me, but something similar did.
Here's what our flue looks like:
Notice the elbow at the top is inline with the wall. Since then I've swivelled it round to be at about 30o to the wall as the steam was making the wall damp and I feared long-term damage.
What I think has happened is that driving rain has entered the flue and made its way down to the fan, where it's shorted the circuits or some other damage was caused. The reason I think this happened is that on the morning it went there was a puddle beneath the boiler. Also I've noticed small damp patches there in the past. I've even seen it drip once or twice from within the main body of the boiler. Obviously last Wednesday night there was a storm and the rain was at the right angle to flood the fan. The more I think about the more it makes sense.
Next time it rains I'll keep an eye on it and see what happens. In the long run I need to change the flue to point downwards.
It was an expensive lesson to learn as it cost £160 to have the plumber who installed it fix the new fan. Not sure whose fault it was though. Is it a design fault or was it the installer's fault for not positioning the flue right? I'm surprised more boilers don't suffer the same fate.