F29 with gas available would generally mean the flame is starting, and being 'baffled out', this would lead to two suspects:
The flue (break down in the flue seals meaning the boiler is burning already spent flue gasses).
Or: (and more likely)
The condense is blocked (the gasses can't escape through the flue because the condense water is backed up into the boiler).
Listen for a gurgling sound while the boiler is firing, which will indicate blocked condense.
Carefree, January 2012