While researching my father's home village I stumbled across this excellent website presenting a detailed history of the Cable Works. It also carries several quite recent photos which illustrate just how important the company once was to Lydbrook, just a shame the buildings were left to crumble. Click on the photos to enlarge them if required.
http://www.forlornbritain.co.uk/ediswan.php

Also see this brief timeline of the company.
http://www.gracesguide.co.uk/Edison_Swan_Cables
http://www.gracesguide.co.uk/Edison_Swan_Electric_Co

UPDATED POST (May 2014)
The "forlornbritain" site appears to be dead at this time (May 2014), but I'll leave the link incase it's restored. Meantime here are some similar photosets of the site now.

http://www.derelictplaces.co.uk/main/showthread.php?t=10745
http://www.derelictplaces.co.uk/main/showthread.php?t=11160
http://www.whateversleft.co.uk/industrial/edison-swan-cable-works-lower-lydbrook
http://www.mark-davis-photography.com/explore/lydbrook-cable-works/

Here is a company history produced with grateful thanks to various sites including the above,

H.W. Smith & Co. established in 1910 as the Electric Wire & Cable Co. at the Trafalgar Works, Lower Lydbrook. In 1912 it moved into new premises on the other side of Stowfield Road near the River Wye and adjacent to the the Railway Junction, to be known as the Lydbrook Cable Works, at first only employing about 40 workers.

http://www.bbc.co.uk/programmes/p02mr7y0
http://www.forest-of-dean.net/gallery/lydbrook_2/pages/page_65.html

When the First World War broke out in July of 1914 the Lydbrook cable works was still incomplete but it was one of only four in the country capable of producing braided electrical cables. The company was awarded numerous contracts by the Ministry of War to produce Army D3 Telephone cables for the field telephone sets bound for the western front. To meet the massive increase in demand the companies workforce rapidly grew from 40 to 650 including many female munitions workers who worked a double shift pattern for the duration of the war. The factory was completed in 1916 with the addition of a powerhouse and adjoining stores and factory building, by the end of the War in 1918 the company had produced over 15,000 miles of electrical cable. With the end of the War, came a slump in business, and in 1920 the Official Receiver was brought in ending Harold Smith's connection with the factory.

http://www.forest-of-dean.net/gallery/lydbrook/pages/page_4.html
http://www.forest-of-dean.net/gallery/lydbrook_2/pages/page_64.html

The factory was then acquired by the Edison Swan Electric Co. about 1925, by then employing some 1200 people, producing Power Line Cables. With the greater resources available the plant at Stowfield further expanded, and was well-placed to help with the Second World War, possessing one of only four machines available for making the lead alloy tube needed for "P.L.U.T.O." (Petroleum Lines Under The Ocean), which allowed fuel to be supplied to the Allied invasion force in Europe from Britain soon after the invasion started. It is believed by some local residents that the Works may also have produced Munitions in both World Wars, altho' this might have been at other factories in Lydbrook ?. It was certainly deliberately attacked by bombers in WW2, as remembered here https://www.sungreen.co.uk/Lydbrook/Wartime_memories_of_Lydbrook.html

http://www.forest-of-dean.net/gallery/lydbrook_2/pages/page_62.html

In the late 1940s, Edison Swan was swallowed up by the Associated Electrical Company, and integrated with the Siemens Cable Works at Woolwich, London. The Cable Works business came to an end in 1966 with the loss of 650 jobs when the factory was bought by Reed Paper Group, and it took on the new name of "Reed Corrugated Cables". It was then purchased by a Swedish packaging firm called SCA, and the site was used as one of several SCA packaging plants around the UK. Production reportedly ceased in 1994.

Also see this photoset, which states
"Near the back of the factory a small railway viaduct crosses the river. The viaduct once carried a private railway siding from the GWR at Lydbrook Junction right into the factory site. The railway long disused has been lifted and the viaduct now carries the foot path to the opposite bank of the river."

Grateful thanks to https://www.flickr.com/photos/normanpreis/3531521603/

More photos of both the Ediswan & the later Reeds businesses, and their employees, in much earlier times may be found here, listed under Lower Lydbrook. http://www.sungreen.co.uk/Lydbrook/_LydbrookPhotos.htm