Do you know what, I thought it was a long shot posting the question, I didn't really expect an answer, except in vague terms along the lines of "likely to be x, y z" etc. RMWEB comes up trumps again!
The problem is that the 12yr old me had the line in the book mentality-I didn't really take notes, except to scribble numbers down on scraps of paper, that then got discarded/lost after the numbers had been underlined in the book! So I have no record of van nos, or even of the list of locos I saw in the works! It's an Instamatic photo, so no chance of getting a van no.
I would imagine it is 163, I can't imagine there being two van trains close together in the afternoon. It is an Up train, but I'm only guessing at the time, so it could easily be 1700 ish, or alternatively, being a Sat & being emcars, maybe the van train took an earlier path? It's definitely an afternoon shot, after leaving the works-I have two or three of the Wizzo & late-lamented 818 Glory around the turntable that come earlier on in the film. We had to travel back to Leighton Buzzard, so we'd have been waiting for an HST to Paddington, which from memory would have been at about 10 past the hour.
Thanks all, these little threads are really satisfying when you can piece a piece of the jigsaw that has been missing for nigh on 35yrs!
cheers N