I had previously found what is a border around the edge of the lawn and thought it looked like it could be a garden railway trackbed...
Well that is now confirmed with this...
(https://www.ngaugeforum.co.uk/SMFN/gallery/159/5581-080426142435.jpeg) (https://www.ngaugeforum.co.uk/SMFN/index.php?action=gallery;sa=view&id=159153)