Hi,
One of Paul's links was "Homemade" which was the key to finding what I was after on DuckDuckGo. A search for "Homemade steam engine from a water pump" led me to this site on Youtube:
He's started with just the sort of pump I'm looking at. Years ago I used to have one like it but foolishly gave it away. I started thinking about it again when I saw one lying on the ground outside the local pump engineers. It's in a bad state but all I need is the crank, piston rod and the cylinder. I'll watch the videos and decide if I want to attempt it as a project.
Regards,
Alan