My 10-year old boiler (conventional, open flue) has started smelling
of gas when it fires up (and recently set my CO detector off). The
flue and ventillation are all fine. I've checked the input and burner
pressures and gas rate - all spot on.

The flames don't look too good - very yellow and look too big. All the
burners etc. have been properly serviced recently and this hasn't
fixed the problem.

regards,
Alan