It's been bitterly cold here this month (lowest -15°c ) when our boiler developed the same F1 fault! The fan would run for a couple of seconds then cut out, run and cut out and so on. Also, while it was running there was a kind of gurgling sound coming from inside the unit.
After standing outside with the hair dryer on the overflow pipe for 20 minutes, there was a thunk as an icicle freed itself from inside the white plastic followed by water which had backed up behind it, and Voila! problem sorted.
Been running for a couple of days now, no problems.
Thanks for your help!
KJ from a very cold North Devon
December 2010