[pedant]
The TV prog referred to a picture of Cholmondeley and Montagu posing with the van shortly after crossing the Scottish border. In the book the same (I think) pic is annotated "at Langbank on the River Clyde". Langbank ain't close to the border.
[/pedant]
'Just pretend to be lovers to give a ring of truth to our letters.' Yeah, right, Monty!