I have drawn a blank at the British Film Institute in getting a copy of the Ro-Railer's publicity images. However thanks to the internet and the freedom of the press in the United States, I attach a link to the open source clip of the movie, as promoted in the US Movietone News.
It was filmed on the Hemel Hempstead Branch, not the SMJR sadly. It trialled there before entering revenue service on the SMJ. It's excellent for me because I could never work out how the road wheels were lifted away from their steel counterparts. Now it has all become clear.