What and where - the answer

Well done, Sue of Indigo Dream. You correctly identified the M25 viaduct over the Grand Union (and the River Gade) at King's Langley.

What a difference the sun makes! The top photo was taken at about half-past eight in the evening of 21st July 2013, while the bottom photo was how it looked at about half-past two three days earlier.

Both pictures were taken from Jubilee as we cruised past on our way to and from the last IWA festival at Cassiobury Park. (And the word "last" could have two meanings, as the IWA seems to have realised that it's not a good thing when a venture supposed to make money actually loses it.)