At the turn of the 80's to the 90's, I was heading through the car park at Northampton castle station when I heard an unfamiliar diesel engine. It was the day before the Bescot open day of that year. We headed over to the noise, to see a 31 coupled up to 55016. They could have been waiting for a path, or laying over for the night. The thing was that the 31 was ticking over, but the deltic was really 'giving it the beans'. Now, I know they're both blue star loco's, but although it was only 50 yards or so in a goods yard, I am still convinced that I saw a preserved diesel move under its own power on BR metals. Does anyone know anything about this, or any other movements of preserved diesels on BR metals?