Hi, I think that the U1 is appearing in the Stockton and Darlington anniversary, for it is carrying No 42 at the front - the 42nd exhibit. It had been finished just in time for that anniversary on 1st July, 1925. It was in workshop grey, after the anniversary, it was repainted in black. In the film it is fairly clearly in grey not black. Also note the people on the trackside as well, which ties in with that anniversary. I hope that this might be of some help.
All the best,
Market65.