She was 16. This week, as she celebrated her 65th birthday, Margaret told the ‘Journal’ she now views this act of casual brutality as a benediction due to the fateful events that unfolded therefrom.
"I lived in Ivy Terrace at the time. We would walk up round the garage and back because there was nowhere to go at that age at that time.
"There was a foot patrol. They had a habit of stopping people and searching them but they had no policewoman with them so we didn't stop. We told them we were going for a walk,” she recalls.
When one of the squaddies struck Margaret with his gun the pain was excruciating but the girls moved on.
“We thought he was looking for attention, showing who was boss," says Margaret.
Sore and bruised Margaret put the incident behind her. But it later transpired the blow had caused some minor muscle deformation.
"I never thought anything about it until I was about 25. That knock grew on the muscle on my shoulder and when I lifted my arm it came up like a big duck egg. I got very inferior about it,” Margaret confides.
She arranged an appointment to get the lump removed. It was a hospital visit that changed everything.
“They took it off and found 200,000 white blood cells. That’s how they discovered I had leukaemia,” she says.
Today Margaret strongly believes things happen for a reason.
"I look at that slap with the rifle now as a blessing. Leukaemia is a silent killer and I wouldn't have known anything about it had he not hit me on that arm.”
Margaret was diagnosed with chronic myeloid leukaemia (CML), a rare cancer that affects the blood and bone marrow. She was sent to Belfast.
"Up in Belfast in 1985 they said I was going to have to have a bone marrow transplant. They had to test all my family. My brother John [McLoone] and my sister Rosemary, were matches.
"They chose John because his red blood cells were very strong. They took bone marrow from his back, from his spine.
"He said he just felt like he had been kicked about a bit when they took it from him. He was delighted to do it. I would have been lost without it.”
After a successful transplant Margaret had to undergo gruelling sessions of radiation and three months in isolation.
"I had to go to Belvoir and got into this wee Perspex cot. They all left the room and I got blasted. Before that I had cut my hair short and then they shaved it. So I didn’t have that trauma because I would have lost it all anyway.
"They took my back to the Royal. When I was in that isolated room for three months my family would come up.
"There was a glass screen. They would talk to me through a telephone. They couldn't bring me anything. If they brought food it had to be radiated. Radiated water that was like sterilised water.”
After this ordeal Margaret got the all clear. She returned to Derry where she lived without any major relapse for a decade and a half. She met her husband Paddy. They wed in 2001.
Celebrating in the Caribbean the newly-weds received terrible news.
"I was on my honeymoon in Havana in Cuba. I phoned the house and they told me they had got a call from the hospital to tell me to get straight up to Belfast when I got back.
"I went to Belfast. They said 'Margaret, it's back'. I had a good 14 years and then it came back.”
The news hit her like a ton of bricks: “It was like a dream. I don't know where I got the strength.”
By this time the treatment of CML had advanced. A new drug called imatinib – invented in the late 1990s and sold as Glivec – had been showing promise in the treatment of leukaemia. Her consultant haematologist Professor Frank Jones was keen to get her started.
"Professor Jones, who did the transplant, was ringing England to try and get me on this new tablet called Glivec, a 'golden bullet' medication for the treatment of leukaemia.
"They put me on that. In between I had to get blood transfusions and they had me on iron tablets because my white blood cells were taking over the red. I was on Glivec for ten years.”
After a decade Margaret was told leukaemia patients who had undergone radiation and bone marrow transplants were now able to come off the medication without any ill effects.
“So they took me off it,” says Margaret. She has been doing well ever since.
Up until late last year she was getting her blood checked every three months to make sure she was leukaemia-free but at her review in December she was told she was doing so well six monthly reviews would suffice.
Margaret was joined by her family and Professor Jones for a special birthday celebration in the Bishop’s Gate Hotel on Monday. For Margaret it was a celebration of life.
"Somebody at Christmas asked me, ‘why are you so happy?’ I said, ‘every day is Christmas, every day is a bonus’,” she says.
Margaret maintains a strong faith and believes had that British soldier not lashed out at her 49 years ago she wouldn’t be here.
“I strongly believe in God. I carry a picture of Clare Crockett in my purse. My key ring is Padre Pio's relic that I've always had that I got down in Knock years ago.
“If that soldier had not been showing off I might be dead now. At that time I had never been in a hospital. I'd never even got my tonsils out.
"I believe I've got a guardian angel. I believe that was meant to happen. It was not a coincidence. That's what saved me. Somebody was looking out.”

1. Margaret Jackson pictured with her brother (and donor) John McLoone and consultant Professor Frank Jones at her 65th birthday celebrations in Bishop’s Gate Hotel. Photo: George Sweeney
Margaret Jackson pictured with her brother (and donor) John McLoone and consultant Professor Frank Jones at her 65th birthday celebrations in Bishop’s Gate Hotel. Photo: George Sweeney Photo: George Sweeney

2. Relatives and friends of Margaret Jackson celebrate her 65th birthday in Bishop’s Gate Hotel. Photo: George Sweeney
Relatives and friends of Margaret Jackson celebrate her 65th birthday in Bishop’s Gate Hotel. Photo: George Sweeney Photo: George Sweeney


4. Family and friends of Margaret Jackson celebrate her 65th birthday in Bishop’s Gate Hotel. Photo: George Sweeney
Family and friends of Margaret Jackson celebrate her 65th birthday in Bishop’s Gate Hotel. Photo: George Sweeney Photo: George Sweeney