Must admit I was a bit worried about it being a pill box, I've been using the defence of Britain overlay on google earth to try to find the location and, as you say, they are all type 24s. I'm wondering if it's some form of air raid shelter, some did have windows high up as in this illustration of one in Birmingham, but why there would be a shelter out in what appears to the middle nowhere I don't know.

cliveh, Eldridges lock seems to have the weir to the right of the lock chamber but, as you say, the locks have been rebuilt since. The print would have been taken in the mid 60s when my parents friends had a cruiser on the river and I don't know when the rebuilding took place.