Devonshire House was never a factory, but rather was the administrative headquarters of the Rootes Group. Photographing this building was a bit tricky as there is a very busy city happening around it.
Devonshire House is situated on the block formed by Piccadilly, Berkeley Street, Mayfair Place and Stratton Street. It's opposite the Ritz Hotel and close to many famous landmarks, including Buckingham Palace.
The easiest way to get there is on the underground. Devonshire House is conveniently positioned directly above Green Park Station. In the A-Z Street Atlas of London, you'll find it on map 77, 1F and on map 145, 2K.
If you have Google Earth, you can see recent satellite images of Devonshire House by opening this .kmz placemark file:
Photographed on 13 May 1998.
Viewed from Mayfair Place.
This last photo was taken in the sixties. It appears to be the corner of Piccadilly and Berkeley Street. The showroom on this corner now houses a Canon (as in office equipment and cameras) dealer. From Graham Robson's Cars of the Rootes Group