In case people are interested in what caused the rattling:

The guy from vokera came today and he found that the flue had come
apart up in the loft! This meant it was discharging flue gases into
the loft and also sucking the outlet gases back into the inlet in
again causing the flame to stutter (loudly) and go out.

He was rather critical of the original installation for not supporting
the flue sections, resulting in them gradually working loose and
separating.

Quite a dangerous fault really... I'm annoyed I didn't think of
checking up there before paying for Vokera to come and diagnose it for
me. Ah well.

Robert