I was told to call Baxi and book a serviceman's visit as he suspected a faulty fan or gas valve, which I immediately did, paying a considerable sum in advance covering a call-out, repair work and all necessary parts.
I'm sorry, this was all resolved long ago and so I had stopped watching this thread. To investigate the suspect test, I called out Baxi's own engineer, who declared the boiler to be well within safe limits. This all seemed to be due to the classic cause of the meter being left overnight in a cold van, and not having been retested recently. Fortunately I hadn't paid the original Gas Safe engineer at that point.