I'd hoped it was the cap for our well - it's somewhere around there - but now I'm not so sure.
It's about 3 inches diameter.
The iron pin comes out with a little persuasion, and then I see this
Inside there's something else rusty, it doesn't show too well on the photo
but it's a stalk with a cone on the top; the cone has a flat tip, and feels as though it's fitted into a bit of rubber - it moves a bit side to side.
The pipe is embedded in concrete. I'd guess it was built in the 1980s, which is the age of the new house next door.
Andy