Tim Don will never forget the pain. The agony coursing through his veins. Arguably one of the greatest ever British athletes — yet relatively unknown — had been reduced to a quivering wreck with a medieval-looking brace known as a halo screwed into his head to heal a broken neck.

Friends and family were just relieved a collision with a car while training on his bike two days before the World Ironman Championships in Kona, Hawaii, in October 2017 was not fatal. Someone in the house, however, was not happy.

‘My three-year-old son was really annoyed because we always used to play wrestling after a bath,’ said Don, the son of former Premier League referee Philip. ‘He didn’t understand what was happening when I came back with this thing on my head. He would charge at me wanting to play but my wife Kelly had to pull him away.’

Remarkably, on Sunday Don will line up in the European Ironman competition in Hamburg — a 2.36-mile swim, 112-mile bike ride and then a full marathon (26 miles, 385 yards) — aiming to return to the biggest stage of all just nine months after almost being killed.

Don, who was born in London but lives in Boulder, Colorado, needs a top-five finish to clinch his place in Kona and lay some demons to rest.

‘I won’t be going to make up the numbers,’ the 40-year-old said, even though his neck injury means when he swims, he can only breathe when turning to his left. ‘I am going to compete.’

Before that fateful day, Don had been on fire, having smashed the Ironman world record in Brazil by four minutes six months earlier.

After three Olympic appearances between 2000 and 2008 ended without medals, he began to excel in the brutal Ironman triathlon.

‘Racing is my life,’ he said. So when the doctors in Colorado surveyed the damage, his plans for a miraculous recovery were laid bare. And it was not pretty. Surgery would fuse the bones together but end his career. So he was given the option of having titanium pins screwed into his skull which are then attached to metal bars, creating the most unwieldy head brace you have seen. One which is worn 24 hours a day for three months, causing excruciating pain.

‘I was so lucky that the doctor in Colorado was an Ironman,’ said Don. ‘I had a broken C2 vertebra, which is found in people who’ve hanged themselves. He said if you want to have an active life with your kids, you have to have the halo. It is painful. My mentality was if a doctor tells you to take some medicine, you take it.’

The footage in the excellent documentary ‘The Man with the Halo’ (produced by award-winning director Andrew Hinton) of the brace being fitted is gruesome stuff.

Don slept bolt upright — but could not do so for more than 90 minutes. His life was put on hold. One of the screws kept coming loose. It was tightened so much it could have punctured the brain and caused irreparable damage.

Yet everyone — including Swiss-based sportswear sponsors On, who immediately renewed his deal for another three years following the accident — had the utmost belief in an incredible recovery.

‘I was just sat there in crazy pain, not being able to do anything,’ he said. ‘And then the kids would come home from school, and they would just carry on as normal. I had to try and be myself for them.

‘My daughter Matilda had brain surgery when she was younger so she understood. Hugo didn’t get it. They enjoyed Christmas, though. They put fairy lights on the halo.’

Don’s wife was vital to his recovery. ‘She went from having two kids to having three. And one was a right grumpy bugger. I couldn’t put clothes on, the halo is covered in sheepskin so she had to sponge-bath me for three months,’ he said.

‘Some of the pain medicine made me vomit, which was so painful. I couldn’t watch television, couldn’t concentrate.’

Recovery was slow but, six weeks after the halo was fitted, it was back to the grind. Don, resplendent in the gym with his metal halo, used an AlterG anti-gravity treadmill, which is part of astronauts’ preparations for space missions.

The relief and emotion when the doctors gave him the green light to finally remove the halo was palpable. ‘On the day I was supposed to get it removed one of the scans wasn’t clear,’ he said. ‘It turned into a four-hour ordeal but finally, thank God, it came off.’

Don ran the Boston Marathon in April in 2hr 49min 42sec and won an Ironman in Costa Rica last month. He cannot wait to test himself against the very best.

‘People talk about how brave I am but I get embarrassed,’ he said. ‘I am only trying to survive. To do what I love and make a living. I don’t think I am anything special.’

Many would disagree.