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THE order “Go! Go! Go!” crackled through the radio.

Princess Diana pushed her foot down on the Range Rover’s accelerator and drove an SAS team into a full-blown assault on the stronghold.

But as our dramatic image shows, the training exercise ended badly with Diana being led to safety after a stun grenade set fire to her hair. The princess and then husband Prince Charles were on a secret visit to the Herefordshire HQ of the regiment – motto, Who Dares Wins – to undergo anti-terrorist and kidnap training in 1983.

Almost 30 years on, their eldest son Prince William and his wife Kate are expected to undergo the same intense training to prepare them for the possible perils of royal life. The couple, both 29, recently took a trip to the HQ to receive safety briefings and meet injured soldiers and the widows of SAS heroes killed in action. It is understood they will soon return to the barracks to take part in a highly-charged mock rescue mission.

In 1983 the exercise, at the former SAS base Stirling Lines near Hereford, almost spelled disaster for Diana.

A stray “pellet” from a flash-bang grenade landed on the right side of her head, setting her blonde locks ablaze.

Only a quick-thinking soldier saved her from serious injury.

Diana and Charles also played the part of hostages rescued from “terrorists” by SAS soldiers firing live bullets from their Heckler and Koch MP5 sub-machine guns in the infamous SAS Killing House building.

Former SAS veteran Rusty Firmin was there on that day and he tells how the couple got the fright of their lives. He also gives an incredible account of what Wills and Kate are likely to face. Rusty, one of the special forces team who rescued hostages in the 1980 Iranian Embassy siege, says: “The SAS training exercise is tough, you can never get used to gunfire and the sound of flash bangs.

“Charles and Diana coped extremely well and I expect Wills and Kate won’t be any different.

“It’s very important that they go through it to prepare for the potential dangers of being taken hostage.

“Charles and Diana were wearing black coveralls with His and Hers written on them. Diana was very keen, she asked if she could drive one of the white Range Rovers for the ­strong-hold assault. At one point, she failed to brake in time and hit a wall which sent her shooting forward.

“By now, we were all around her hurling flash-bangs which we use to disorientate terrorists.

“The princess left her driver’s window down for some reason and a burning pellet from a flash-bang landed in her hair which started smoking. She started shouting and patting her hair.”

Diana screamed to her watching husband: “Oh, Charles, my hair. My hair.” Rusty, 61, says: “I ran over and started to pat her hair out with another guy. One of the Ruperts, our name for officers, put his arm round her and she was taken away to recover. Luckily she saw the humorous side of it later.

“It was a shock for all of us when we realised how close we’d come to setting fire to her.” As the visit was secret, Diana got her lady in waiting to trim her hair back into shape before she left so no one would realise what had happened.

Charles and Diana joined the SAS for tea and coffee and a de-briefing after the dramatic exercise.

Then came the real ordeal. They were taken to the regiment’s notorious Killing House – or Close Quarter Battle Building – where troops simulate the rescue of hostages by killing dummy terrorists with live ammunition. The Killing House is designed just like a regular building, with furniture, pictures, toilets, etc.

It has special rubber-coated walls to absorb bullets, extractor fans to clear out gun fumes, and video cameras to record the action in the rooms. And each room has at least one metal target.

The exercise was staged to school the royals in what would happen if they were ever seized by terrorists – and how to react when the SAS moved in. Wills and Kate will have to experience this, too.

Charles and Diana, who died in a 1997 Paris car crash, sat at a table with two men telling them what to do.

Rusty says they were surrounded by cut-out figures of terrorist Carlos the Jackal. And when the order was given, four blackclad SAS troops, armed with assault weapons and wearing gas masks, poured into the large room.

A flash-bang grenade was thrown in and, without taking a breath, the soldiers dispatched the targets in a hail of bullets. Diana covered her ears as bullets ­whistled around, while Charles recoiled in shock at the noise and action.

The SAS team lifted the pair out of their chairs and bungled them to safety. Rusty says: “It was the perfect hostage rescue, we called it a snatch operation. The noise is deafening.

“The prince and princess behaved brilliantly. They were told that if ever they were taken hostage, it was vital they should know how to react when we come. The last thing we want is for a hostage to panic and run into the line of fire. The prince and princess were told either to remain still, or hit the floor.”

Rusty insists it is now more important than ever that Wills and Kate undertake this type of training.

He says: “They can hear the gunfire, see the flashes. It will give them a sense of reality, they will know what to expect. During the rescue Wills and Kate will be man-handled and taken out of the area while the threat is eliminated.

“When Charles and Diana were rescued they were frightened. It takes just seconds for all this to happen.”

Before taking part in the rescue, Charles signed a humorous typed letter which still hangs at the SAS HQ.

It reads: “Should this demonstration go wrong I, the undersigned Prince of Wales, will not commit B Squadron 22 Special Air Service Regiment to the Tower of London. Charles.”

- Rusty Firmin's new book 'Go! Go! Go! The SAS. The Iranian Embassy Siege. The True Story' is available on Amazon . To read more about Rusty go to www.rusty-firmin.com .