That's an interesting photo. To me it looks like a Dotted Bee-fly (Bombylius discolor), less common than the Large Bee-fly (Bombylius major). You can tell it’s a B. discolor by its darker brown end of abdomen, as seen in this photo. You can also compare the two species in these side-by-side photos here and here, although in your photo you can’t see the main character to make an ID – the wing pattern, because it's taken in flight. Body colour should suffice.
I hope this helps. Thanks for sharing this.
Thanks for the reply.
I've had a look at the links you provided and also found another site that describes the way they move about and it's all spot-on. A three-year mystery solved - I wish I'd found this forum earlier!
Shame they're bee parasites - I'd thought they were quite sweet until now.