Hi scubi, yes I did have a look at the old-maps site, the half-moon/kidney shaped feature is in the right place for what I am on about - it is marked 'sluice' (what does this mean?) and appears on the map to be a pond? But it is on top of a hill?? This puts me in mind of the Penton at Pentonville Rd, North London - formerly sacred hill (supposedly) and latterly a reservoir??

The maps after the construction of the railway line omit it - a pond that was drained...?

On the 1894 map it seems to have some contour marks (couldn't zoom so am not sure) around it, so not a pond but a raised 'feature'?

tjj - thanks for your evocative post, yes it is special round there, isn't it - just walking 5 minutes away from the road (on either side) can take you into some quite magical scenery. And how fantastic to see a kingfisher!