Ged,
just had the same fault - doubt its the fan as there are problems it seems with the relays on the PCB.
See details below as added this info to another string within this site earlier
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Just had a fan/flue light flashing on my 105e
primarily checked the flue for blockage (you never know) gave the unit a good clean/vac out
Checked power to Fan when hot water demanded - no output
Still no joy
Ordered replacement PCB - Part No: 5112380 (was 248074)
found this to be an upgraded version - better it seems
off ebay BNIB Item number: 150173291687 for reference price
£48.50 inc P&P
Fitted - (ensure you switch to reset for 3 secs before returning dial to On position) and that power to the unit is isolated - off at fusebox if not on dedicated trip/circuit.
If unsure seek professional help!
Problem Solved
Have my board at work to tinker with and repair for a spare
suspect the Fan relay - but struggling to find schematic details
so will probably replace both suspects!
hope this helps Paul
hfm@ntlworld.com
Paul Hawksworth
October 2007