the above is the Townsman model, the below are Scarabs
In 1934, Scammell produced the 3-wheeled 'Mechanical Horse', designed to replace horses in rail, postal and other delivery applications. This featured automatic carriage coupling and the single front wheel could be steered through 360 degrees. It was sold in 3- and 6-ton versions. The 3-tonner was powered by a 1,125 cc side-valve petrol engine and the 6-tonner by a 2,043 cc engine.
In the late 1940s, the 'Mechanical Horse' was superseded by the Scammell Scarab, with similar features but a much less angular cab and now with a 2,090 cc side-valve petrol engine in both models and a diesel version with a Perkins engine.
In 1967, the 'Scarab' was replaced by the 'Townsman', which had a fibre-glass cab.
Photos from Cardisiac: http://www.forum-auto.com/automobiles-mythiques-exception/section5/sujet225087-455.htm