Thanks for the ID Alf, the penny drops. I've glimpsed a bee on occasions coming from the small patch of ivy carrying what I thought was a part of a leaf,
and shoving it under a flower pot, with a raised base.
The problem is that its one of many with trainee bonsai trees in, and of course they need watering.
I did move the pot elsewhere and replace it (exactly) with the same type just with loose soil in it, but it wasn't fooled!
Finally I returned the pot to its original position. and left the empty one, also, in the hope that it would go to there instead.