Andrew Gabriel presented the following explanation :
My Keston does that every 24 hours of non-use, according to
the manual. In 16 years, I only heard it do it once, but you
need to be in the bathroom at the time, and not having a
shower or anything else noisy, in order to hear it.

My boiler is in a kitchen cupboard, which makes it noticeable when it
is quiet in the kitchen. I noticed it this morning again, at 09:45. It
is set to allow CH, but the stat is set to 15C so it should not be
called in this warmer weather at all. The last time it was called to
provide any heat was yesterday, for HW - I had a bath, then Jean had a
bath in the afternoon and it was timed to come on heat HW at 16:20. So
probably there would be no call for the boiler to fire any later than
17:20 yesterday.

I have not read the manual thoroughly, there is a lot of it, but I
didn't spot any mention of it doing an occasional purge, so I have
emailed Vaillant to ask them about it.