Oh, and just to say, after a minute or so sitting on Mr BW's palm to orientate itself, it flew off, perfectly normally and happily, into a nearby tree, and then on from there.

A Wood Warbler? Green above, white below, not the finch beak of a greenfinch.

Long time London-based lurker via Diamond Geezer, very occasional contributor, I admire your joint stamina over the past half a decade or so.

Juvenile greenfinch?

I think it's a chiffchaff.
Years ago, when I used to do pub quizzes, I learnt a whole load of collective nouns. Most of them purely artificial, invented just for the sake of it, I think.

Gordon - that is a definite possibility. Thanks for decloaking to assist!

Debster - that was my first thought, but it doesn't have the right beak. The feathers are ruffled like a juvenile's, but it flew too well.

Tim - that is another possibility.

I found a page that does willow warbler v chiff chaff discrimation, but not wood warbler v chiff chaff. It's one or the other I think...