Chris Hogg wrote:

We get this after the water company has been doing work on the mains
supply pipes. Air gets into the pipes when they open them up, and when
re-connected the air is trapped in the pipe (we're at the end of a
mile-long uphill stub, which tends to empty when the main is opened
and doesn't get flushed when they re-connect).

There should be a "wash-out" at the end of such runs to allow the water
company to flush the system.