He had been in the US for a medical conference in March 2004, reporting on topics including deep vein thrombosis. The night before his journey home, Mark had a few drinks.
The flight was long, with a six-hour transit in Washington DC because of delays. "Door to door, it was a 20-hour trip," says Mark from north London.
"I arrived at Heathrow feeling as though I had slept quite badly. I felt a cramp in my left leg, and it got worse over the next few days."
It was when Mark began limping because of the pain that he got medical attention. And that was only at the insistence of his wife, Jill.
"My leg was hot and swollen," says Mark. His GP diagnosed DVT and Mark was sent to the Whittington Hospital in north London, where he received an ultrasound that found a blood clot stretching from his calf to his mid-thigh. "The clot had spread quite a bit," says Mark.
He was kept in hospital for a few days. He received daily injections of the anti-clotting treatment heparin, and was put on a course of warfarin tablets to prevent further blood clots.
"It's ironic that I got DVT after attending a conference on it," says Mark. "I should have known better and gone straight to my GP."
Doctors were puzzled by Mark's DVT. He didn't have any of the risk factors: he was male, a non-smoker, was a healthy weight and had no family history of DVT. They said the long flight, dehydration and a lack of movement contributed to the DVT.
After six months, Mark stopped taking warfarin because doctors thought he was unlikely to have another clot.
But he did. In June 2007, Mark developed pain in the same leg. And the consequences could have been far worse.
"It was a dull muscular pain, which came and went," says Mark. But even then, despite having experienced DVT, Mark didn't realise what it was. "I think I was in denial," he says.
After a few days, he developed severe chest pains. He went to hospital, where a spiral CT scan found five blood clots in his lungs, known as a pulmonary embolism.
"Part of the clot in my leg had broken off and got into my pulmonary artery," says Mark. It's a potentially fatal condition.
He was in hospital for four days, where he was treated with heparin and warfarin. "I was very lucky," says Mark. "I was healthy, and that's why I'm still alive.
"With the first DVT, I just thought I was unlucky. But with the second one, it was the first time I had come face to face with my own mortality.
"Doctors blamed the pulmonary embolism on the fact that I was sitting down for hours on end at my desk without moving, and not drinking enough water," he says.
Mark is now in a high-risk category and he'll probably need to continue taking warfarin for the rest of his life. When flying, he wears compression stockings (also called flight socks) to boost the blood circulation in his legs.
"During the flight, I make sure I drink water regularly, don't drink alcohol or coffee, and exercise," he says. "I try not to fall asleep either, because that means I'm not moving."