The sunbeamsidevalve.com site remains online, but almost all of the images are missing. It has not been updated for a decade, as of November 2021.
An archive of the site is available which displays much of the original imagery.
sunbeamsidevalve.com at archive.org
The site credits a number or sources, including:
S J Mills, 1994, 'AJS of Wolverhampton'. Published by the Author.
Robert Cordon Champ, 1980, 'The Sunbeam Motor Cycle'. Haynes Publishing.
Robert Cordon Champ, 1989, 'The Illustrated History of Sunbeam Bicycles and Motorcycles'. Haynes Publishing.
Wolverhampton Chronicle, 23 November 2006, 'Sleek Sunbeam is shining example of craftsmanship'.
John Marston Ltd., 'Sunbeam Motor Cycles' Catalogue. (various years)
John Marston Ltd., Sunbeam Spare Parts List. (various years)
'Motor Cycling' magazine, January 14, 1931; September 7, 1932.
Roy Bacon, 1986, 'British Motorcycles of the 30s'. Osprey Publishing
Express & Star, 1995, 'Images of Wolverhampton'. Breedon Books