Thanks for the reply, Vanoord.
The locos in the photos appear to be small Ruston diesels, probably 20DL type.
The ex-Penrhyn one was a commercial light channel rail chassis by Muir-Hill Service Equipment Ltd, with the engine, bonnet and radiator (and possibly seat) from a 1920s Morris motor car. The steering column had probably been dispensed with as well.
Penrhynman