"She did not see the engine fall away but it was described to me that it fell into a flooded gravel pit about a mile from their house, in the area of Thorpe Park and the M25/M3 junction nowadays"
If I remember correctly, it was known as Marco's Pit (or DeMarco's), and it is located to the east of Chertsey Lane, close to the main entrance to Thorpe Park. The pit is still there, used as a fishing lake, and a bit of a "sh*thole", according to a recent description I've received!