Cravens
Established in 1853 as an engineering firm the company would eventually diversify into building rolling stock and the DMUs would be built in its premises at Darnall in Sheffield. The rolling stock part of the company was taken over by Metropolitan-Cammell in 1966.
They built 405 DMU vehicles for British Railways:
Online company histories:
Wikipedia
Grace's Guide to British Industrial History
In the image a Class 114 passes the Cravens premises at Darnall in 1979. Steve Davies Collection.
Today
All signs of the factory have now gone. This youtube clip shows the site in 1993 before demolition to become a housing estate. It is partly filmed from the footbridge over the railway line between Acres Hill Road and Acres Hill lane, as in the Class 114 image.