Sacha wrote:

Slow worms, grass snakes, frogs, some other kind of grass hopper? Ground
nesting bird? Borer bees/wasps?

Thanks for the replies everyone. This little mystery looks like it will
remain unsolved for the moment as there are so many candidates for the
noise. I'll see if I can find the sound of each creature suggested on
the web. I think the sound may be some sort of mating call or territory
thing as two of them were taking it in turns to make the sound, one each
side of me. They seem to like the hot sunny weather we've been having here.

--
David in Normandy.
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