After a bundle of emails - but no comments here - here are the answers to the post below. Most respondents were 90% correct only falling at the source of the photo.
The vehicles(?) are GWR Iron Minks (with wooden doors). The location is Machynlleth goods yard were they were used as buffer-stops. Note what looks like bridge rail under the bodies. The photo was lifted from the Clubs and Societies pages of an early 70s RM and credited to one Robert Fysh. I would think that the photo was fairly recent to the publication date judging by the lorry in the background. The caption states 'Note the drawhook for pulling stops back after successive shunts.' I assume the vans were a budget version of sliding buffer stops.
Thanks to those that responded.