Thank you for looking at this for me. None of them seem to move - there are quite a few on each carrot leaf and they just sit there. I photographed one a few days later and it appeared to have a hole in it as did quite a few others. I wasn't sure if it had been attacked by something but presumably the hole could have been caused by something getting out.
They seem to appear quite suddenly - I'd been watching the carrots for signs of pests as we've had these before. We seemed to go from clean leaves to lots of these pests in a very short time.