Sacha wrote:
I emailed a pic to David Poole and got this reply:

"It is an Acer and it is almost certainly the highly variable Acer
negundo - and the images seem to portray mainly male inflorescences
they are very typical of the species.....

That's very interesting indeed. I've followed up all the other suggestions
very carefully, and they all appear to be blind alleys. This one seems
about spot-on, except two other references descibe the male floiwers as
corymbs; which I associate with an upright habit and all the florets forming
a plane. These are variable length and dangling. However these photos from
authorative sources seem to have clinched it:

Male:
http://www.funet.fi/pub/sci/bio/life...negundo-1m.jpg
female:
http://www.funet.fi/pub/sci/bio/life...negundo-2f.jpg
The parent web site also gives an English common name: Ash-leaved Maple,
which is nice

I also found, following your clues, Acer Negundo Californium
http://elib.cs.berkeley.edu/cgi/img_....+californicum
where the florescences are much more exuberant then my little delicate
thing - though the picture of the main tree shows the same sparse leaf cover
even when fully developed.

So, a male Acer Negundo (Box Elder, Ash-Leaved Maple) will do for me. Now,
the next interesting bit. How did it get here? The examples above suggest
a north american range, on the atlantic seaboard, although two of the photos
are from Finland. Is mine a weed, or did a previous owner purchase it - and
if the latter, was it once sold commonly in garden centres? It's perfect
where it is, slow growing and small, and I like it a lot.

For those who joined the group recent, David was - and I hope he will
be again when less busy - an immensely knowledgeable member of urg.
I've never known him wrongly identify anything.

My thanks to both of you.