By Mark Ward
Technology correspondent, BBC News
By the end of WWII, 11 Colossus machines were in use
Bletchley Park is best known for the work done on cracking the German codes and helping to bring World War II to a close far sooner than might have happened without those code breakers.
But many believe Bletchley should be celebrated not just for what it ended but also for what it started - namely the computer age.
The pioneering machines at Bletchley were created to help codebreakers cope with the enormous volume of enciphered material the Allies managed to intercept.
The machine that arguably had the greatest influence in those early days of computing was Colossus - a re-built version of which now resides in the National Museum of Computing which is also on the Bletchley site.
Men and machine
The Enigma machines were used by the field units of the German Army, Navy and Airforce. But the communications between Hitler and his generals were protected by different machines: The Lorenz SZ40 and SZ42.
The German High Command used the Lorenz machine because it was so much faster than the Enigma, making it much easier to send large amounts of text.
"For about 500 words Enigma was reasonable but for a whole report it was hopeless," said Jack Copeland, professor of philosophy at the University of Canterbury in New Zealand, director of the Turing Archive and a man with a passionate interest in the Bletchley Park computers.
Bletchley employed thousands of code breakers during wartime
The Allies first picked up the stream of enciphered traffic, dubbed Tunny, in 1940. The importance of the material it contained soon became apparent.
Like Enigma, the Lorenz machines enciphered text by mixing it with characters generated by a series of pinwheels.
"We broke wheel patterns for a whole year before Colossus came in," said Captain Jerry Roberts, one of the codebreakers who deciphered Tunny traffic at Bletchley.
"Because of the rapid expansion in the use of Tunny, our efforts were no longer enough and we had to have the machines in to do a better job."
The man who made Colossus was Post Office engineer Tommy Flowers, who had instantly impressed Alan Turing when asked by the maverick mathematician to design a machine to help him in his war work.
But, said Capt Roberts, Flowers could not have built his machine without the astonishing work of Cambridge mathematician Bill Tutte.
"I remember seeing him staring into the middle distance and twiddling his pencil and I wondered if he was earning his corn," said Capt Roberts.
But it soon became apparent that he was.
"He figured out how the Lorenz machine worked without ever having seen one and he worked out the algorithm that broke the traffic on a day-to-day basis," said Capt Roberts.
"If there had not been Bill Tutte, there would not have been any need for Tommy Flowers," he said. "The computer would have happened later. Much later."
Prof Copeland said Tommy Flowers faced scepticism from Bletchley Park staff and others that his idea for a high-speed computer employing thousands of valves would ever work.
Colossus kept valves lit to ensure they kept on working
"Flowers was very much swimming against the current as valves were only being used in small units," he said. "But the idea of using large numbers of valves reliably was Tommy Flowers' big thing. He'd experimented and knew how to control the parameters."
And work it did.
The close co-operation between the human translators and the machines meant that the Allies got a close look at the intimate thoughts of the German High Command.
Information gleaned from Tunny was passed to the Russians and was instrumental in helping it defeat the Germans at Kursk - widely seen as one of the turning points of WWII.
The greater legacy is the influence of Colossus on the origins of the computer age.
"Tommy Flowers was the key figure for everything that happened subsequently in British computers," said Prof Copeland.
After the war Bletchley veterans Alan Turing and Max Newman separately did more work on computers using the basic designs and plans seen in Colossus.
Turing worked on the Automatic Computing Engine for the British government and Newman helped to bring to life the Manchester Small Scale Experimental Machine - widely acknowledged as the first stored program computer.
The work that went into Colossus also shaped the thinking of others such as Maurice Wilkes, Freddie Williams, Tom Kilburn and many others - essentially the whole cast of characters from whom early British computing arose.
And the rest, as they say, is history.