Kingfishers?

'Unmistakable bright blue.'  'Fly rapidly.'

Without a doubt, I saw one while standing eating a bar on Burnsall Bridge.  It flew across the water and perched on a wall below the path.  I kept an eye on it as people approached and it was quick to scoot off, luckily, towards me and up to the bridge.  I had a little peer over the edge to see if I could see it but nothing... until I left to go and peered slowly over the other side as it was already scooting off - it had heard me coming!  

But, it's possible I identified another on the way home.  I didn't clock him at first because he was silhouetted against the low sun but there were some clear identifying features as I peered through the light including:

'Unmistakable bright blue.'  'Fly rapidly.'