As for the type of vehicle I'm building, GB was closest in identifying the vehicle as a Leyland lorry, although Adrian was the one to spot that it was articulated. The final missing piece of identification is the model of the lorry, which the title of this post should help with.
The model (Langley Models number E20) is supposed to represent a Leyland Beaver flatbed lorry from somewhere in the period 1949 to the 1970's, which fits perfectly with setting the layout in 1955.
I haven't decided on the full colour scheme for the vechicle yet, although I am leaning towards the same as the example shown on the website of a red body, black underframe, with shiny metal details (the bumper, radiator etc.). As you can see from the photo I've only got as far as the basic grey primer, and there is still some work to do filling gaps between the castings as well (most obvious around between the roof and cab walls but I can't do this until I've painted the inside of the cab and glued down the roof). Even in grey primer though I'm liking how it looks so hopefully it will be a good addition to the layout.