While I as back in the UK the other week I grabbed a handfull of B & W Prints to scan when I got back to Ng.
This lot are dated 20th March 1967 and where taken between Chiltern Green & Harpenden Junction.
They note that the Tartan Arrow was a test train. Looks (if my memory is correct) like it is on the Up slow lines.
I hope these are of interest to some one.
Trev.