Pressed Steel (Linwood)
This firm founded in 1926 is better known for producing body panels for cars. It took over the Beardmore Factory at Linwood (outside Paisley in Scotland) where it produced railway vehicles during the modernisation period. The DMUs were:
For British Railways they also produced Mark One carriages, the Class 303 EMUs and many steel wagons. There was also an announcement that they would build Cross-Country sets for the Western Region, this order would be switched to Swindon Works.
In 1967 the Linwood plant was taken over by Rootes Chrysler (the Pressed Steel plants at Cowley and Swindon had been acquired by the British Motor Corporation), it would then produce car panels for a new Rootes factory built next door. They would close in 1981, the Pressed Steel plant is long gone and the site is now the Phoenix Business Park.
The aerial photo shows the factory in 1950, the image is from http://canmore.org.uk/collection/1451931 where a larger version can be found.
Online company histories:
Wikipedia
Grace's Guide to British Industrial History
Britian by Car
Some company archive material is held by Renfrewshire Libraries (search here for 'Pressed Steel'), when I last visited there was nothing relating to DMUs.
The National Archives catalogues other Pressed Steel documents help at places around the country. One from their own files from 1959 titled "Two-car Inter-city diesel units: development by Pressed Steel Company" contains correspondence about Pressed Steel expressing an interest in offering a two-car inter-city diesel for trials which would incorporate new methods of design and construction, using mild steel, which was anticipated to offer a reduction in weight without increase in cost. Nothing came of this.