Have you got a link to this 2+10 HST cab ride video?
Here you go:https://youtu.be/mvfZLFESS3o
At about 3:20 camera chap says something like "oh, we've got 10 on have we, one of the long ones?", driver (the south wales voice) just about audibly replies in the affirmative. I also spotted in the first couple of minutes a 2+7 and a 2+8, so set lengths clearly weren't as, er, set as they are now. I seem to recall a subsequent conversation about acceleration performance later in the film where the 10 trailer cars were mentioned again - not really enough time to watch all the way through right now!
Worth a watch anyway, pre-privatisation, pre-electrification & remodelling of Paddington and also before the demise of the rather fine Paxman Valenta.