Found it after a bit of a struggle: Hover fly Helophilus pendulus. Collins says it likes sunbathing on waterside vegetation.

These are good pollinators too.

The larvae of some species eat aphids, and other plant pests.

Thank you Stera, Loosestrife and Tug. I have lots of hoverflies and I know that they are definitely friends. I don't think I've ever seen one basking, though, and I didn't recognise it. The markings are very striking.

They are - it was easy to eliminate masses of little waspy look-alikes from those vertical stripes!

They will sit in the sun to get their temperature and metabolism up to " hovering " speed. As the days get cooler you will see this of every insect air and land.

I had no idea there were so many different types of hoverfly. The ones I have usually seen are smaller than the one in the photo.

I know that the male bumblebees that "roost" overnight outside the nest can't fly until their bodies reach a certain temperature. Fascinating, isn't it?

Yes it is fascinating and all of it is to be found within a few steps from one's home.

Yet another dimension to having a garden. How lucky we are.