These "auxiliary units" hid in ready-made underground burrows as the tanks rolled overhead, emerging by night to destroy and, where necessary, to assassinate. By some accounts, they were even authorised to execute collaborators.
Recruits were told to keep their secret for life, which might not have been very long. "Life expectancy, in the event of an invasion, was about 14 days," says Geoff Ratcliffe, who joined at 17 while waiting for his call-up. Many members keep their silence still. Enthusiasts working to develop a British Resistance Organisation Museum, based at a former USAF airfield in Parham, Suffolk, have traced only about a third of the 400 members still thought to be alive. In July 1940, with invasion thought to be imminent (the commander of the home forces thought it would be on 9 July), Winston Churchill authorised the creation of a force of "stay- behind units", made up of seven or eight-man fighting patrols. Colonel Colin Gubbins put the scheme together.
"It was very unusual," says John Warwicker, a retired Scotland Yard officer, and dedicated researcher into the units. "A group of stay-behind civilians who were trained by the military to a very high standard of SAS-type operation, with explosives, timing devices, firearms and dirty tricks."
Starting in Kent and Sussex, where the officer in charge was Peter Fleming, brother of Ian Fleming, the organisation spread along the East and South coasts. About 400 underground "operational bases" (OBs) have since been identified, some still in existence. About 4,500 people were trained, although no more than two-thirds would have been involved at any time.
"They picked men who could live off the land, who were exempt from military service by virtue of age or occupation, who were resourceful, and who could be trusted to keep their mouths shut," says Warwicker, who first learned of the units from a neighbour who had been a member.
Geoff Ratcliffe was a founder member of the unit in East Bergholt, Suffolk, along with three farmers and a factory worker, whose occupations kept them out of the forces. All were members of the local Home Guard, and that would remain their cover.
Ratcliffe remembers being given only a vague idea of what was to come. He was shown the OB, a hut buried in the ground in a wood off the A12, where the patrol would hide itself each weekend. At the organisation's headquarters at Coleshill, near Swindon, he was taught to destroy tanks with plastic explosive and "sticky bombs", which you "lobbed underarm" from close range.
He was given no illusions about his own chances in the event of an invasion. "We would have been found and exterminated, no doubt," he says. "There were no long-term career prospects."
Don Hanscombe was a farm manager in his early twenties when he joined. He was recruited in a pub by a member of the local fighting patrol. The Home Guard was supposed to be his cover, but they knew nothing of his activities. So when he was stopped by a policeman, while armed with "a revolver, a pistol, a tommy gun and a sten gun", it took a while before anyone could vouch for him. "Later on, we were issued with a piece of paper saying `No questions should be asked of the person bearing this'," he says.
Once in their bunker, they would have been on their own. "We were pledged to fight on, with or without orders," says Hanscombe. As the threat of invasion receded, from 1942, the auxiliary units were used for experiments in undercover warfare. Colonel Gubbins himself established the Special Operations Executive (SOE) to work with resistance movements across Europe. In 1944, when Geoff Bradford, then 17, was recruited to a fighting patrol in North Devon, there was a fear that the expected invasion of France might lead to an immediate German counter-invasion. Bradford recalls being trained in close combat and issued with a Fairburn-Sykes fighting knife, a nine-inch commando dagger with a double-sided blade.
Roy Bradford, Geoff's elder brother, had been an auxiliary units training officer. By D-Day, he had graduated to the SAS. He then joined the French resistance behind German lines. On 20 July, he was killed in an attack on a German convoy. "There were several like him," says Geoff, one of the surviving members featured in a Radio 4 documentary.
The SAS had recruited extensively from the auxiliary units. "The training wasn't entirely wasted," he says. The auxiliary units have been a long time emerging from the dark. When they stood down, in late 1944, the men were told their existence would never be acknowledged. But 50 years later, a first reunion of members was organised.
In 1996, the Ministry of Defence conceded that the men's service could be acknowledged, so that they might apply for the 1939-1945 Defence Medal. Not all were deemed to qualify, however, and they continue to seek recognition for their service.
It is interesting to speculate on what would have happened if the worst had happened. John Warwicker says: "It would have been very effective behind-the-lines resistance. It couldn't possibly have caused the Germans to fall back. But they would have been a great nuisance."
It would have meant leaving their loved ones, without any explanation for their disappearance. It would have meant going into a hole in the ground and, probably, not coming back. How many of these volunteers would have gone through with it?
"Your question," says Don Hanscombe, "is a pertinent one. One never knows, of course. But I think that 99 per cent of them would have had a go."
`Hidden Heroes' is on Radio 4 this evening at 8pmReuse content