It's that time of year I suppose, although this one is about the hot
water, not the heating.

It's an Ariston Microgenus. It seems to have developed an intermittent
fault whereby sometimes when you turn on a tap the hot water just
doesn't get hot. While the hot tap is running there seems to be a lot
of churning coming from the boiler, which I assume is the pump
running, but neither the fan nor the burners come on. The "fume
sensor" LED comes on until you turn the tap off, whereupon the light
goes out again. This fume sensor LED seems to be a mystery from the
research I've done. The manual doesn't say what exactly it senses. And
I don't see how it can be sensing "fumes" because the burners aren't
lit!

Any immediate ideas as to what would cause this to happen
intermittently?

Cheers!

Martin