StripeyKnickers, could you mean Ride a Wild Pony, a book by James Aldridge? Mine had a photo of a ragged boy riding a palomino pony bareback down a dirt track on the front.
My query is: has anyone come across a short story in a book of ghost stories for adults which features a man walking up One Tree Hill in SE London, and going back in time as he does so? I got the collection out of my local library a couple of years ago, but cannot find it again, and nor can the library staff!
Or (even more vaguely), a children's book which included a pair of children on a train at one point, and the boy had a pet rat and was explaining how chocolate is bad for them - to a girl, I think. Not much to go on...