Looks like one of the many hoverflies, [ a true fly] possibly a common dainty but I cant see the abdomen colours clearly.

It's probably beneficial. The larvae of most hoverfly species eat pests, such as aphids.

It looks like a slimmer version of Xylomyia maculata?

This site: https://en.wikipedia.org/wiki/Xylomyidae

seems to say there are many species.

Apparently there is even a wasp that mimics (Xylomyia?) Xylomya czekanowskii - see this site:

http://www.mallochsociety.org.uk/latest-updates/?currentPage=2

That's a huge relief. I thought it was a menace of sorts. Thanks all.