Interview Craig JohnstonStuart Jeffries talks to the former Liverpool player and football boot designer who remains disarmingly candid about his limitations as a player and modest about his achievements as an inventor

"I was crap," says Craig Johnston disarmingly, as he teases his luxuriant locks into a ponytail at his penthouse flat in London's Chelsea. "You have no idea how crap I was. Even when I was playing for Liverpool, I was the worst player in the best team in the world."

In a way, though, that very perceived crapness has made the 43-year-old former professional footballer the man he is today - a successful businessman who has just been shortlisted for the Designer of the Year award on account of his invention of a revolutionary and extremely knobbly football boot called the Predator, of which more later. But first let's go back to Johnston at his crappest, when he was a 15-year-old who had just flown 20,000km from Australia for a trial with Middlesbrough FC.

"In my first youth team game, we were playing Leeds United and we were losing 3-0. The manager at the time was Jack Charlton and in the dressing-room at half-time he was seething. He had a go at everyone and then he said to me: 'And as for you, you kangaroo, you can fuck off right now. You're the worst player I've seen in my life'."

Johnston was actually born in South Africa and only later raised in Australia, but such nuances were probably lost on Charlton at that juncture.

"I remember I burst into tears and that night I packed my bags. Then I phoned my mum and I couldn't tell her the truth because my parents had sold the house back home to pay for me to come here, so I told her, 'Big Jack loved me'."

For the next six months, Johnston hid from the wrath of Jack - "I just cleaned first-team players' boots and cars, and stayed out of his way." But there was more to it than that. As he recalls in his autobiography, Walk Alone, "It came to a simple choice: either learn the game properly or get the next flight home."

But he couldn't get the next flight home for fear of shaming his family. After all, his father, Colin, had aspirations to become a professional footballer in England or Scotland, but had failed to do so. Before he returned to Australia, though, he visited Craven Cottage with his wife who was pregnant with Craig, and made a seemingly strange prediction. "He said to her: 'My boy will play here one day'."

So Craig Johnston was obliged to become a footballer. In any case, he wanted to triumph over his childhood illness. When he was six he had a form of polio called osteomyelitis, but his mother refused to have his left leg amputated as doctors wished and instead sent him to the US for specialist treatment. He shows me a photo of himself playing for Liverpool and points out the huge scar running down his left thigh, the result of an operation that saved his leg. "With all that sort of baggage, there was no way I could let myself fail."

Johnston, then, had something to prove, but didn't yet have the skills to do so. How would he get them?

"I have always had a creative mind. I wanted to be an architect and I used to design my mum's kitchens. I have realised since that design is about solving problems and the problem that I had was that the football was the perfect object, so the problem was the imperfect thing that was kicking it." You mean you? "That's the fella."

So, unknown to the rest of the club, Johnston devised himself a training regime and devoted himself to it single-mindedly.

"I would get garbage cans and put them in a row and run through them with a ball in 10 seconds. If I touched a can, I would start again. I would put on a blindfold and sense the football with my feet as I dribbled. I put two small crosses on the wall and hit them with my left and then my right foot. If I was good and concentrated hard it would take me three hours. If I was poor, it would take me six hours. It always seemed to be bitterly cold and wet in Middlesbrough."

The training paid dividends: he was recruited into Middlesbrough's first team and scored in his debut match against Everton. "I remember reading Charlton saying, 'I always knew the kangaroo would bounce back.' We've met up since and had a good laugh about it."

Five years after his dressing-room dressing-down by Big Jack, the 20-year-old Johnston was snapped up by Liverpool FC for a then record fee of £570,000. "For a day I was the most expensive footballer in England, then the following day Liverpool bought Mark Lawrenson for £900,000."

It was, from 1981 to 1988, a brilliant career, during which time he made 224 appearances for the club, scoring 39 times. He was an integral part of a team that won the English football league five times, and both the FA Cup and European Cup once.

"I was surrounded by hugely gifted footballers - Ian Rush, Graeme Souness, Kenny Dalglish, Alan Hansen. My job was essentially to win the ball, lay it off to them and make myself available to receive it later.

"I realised that I wasn't one of those instinctive players like Beckham or Maradona. I knew I wouldn't be great, but I also knew that I would have to analyse what other players did instinctively to keep at their level."

It was in his career-saving analyses of what greater players did instinctively, that Johnston found the ideas that were to grow into the Predator, and also into a software system that he developed called Supaskills, which analyses a player's performance to improve his game.

Johnston did play at Craven Cottage too, just as dad had predicted.

"I scored too! I loved playing for Liverpool, especially the atmosphere in the city. It was a place where one minute a total stranger would come up to me, hug me and say I love you, and five minute later someone would wind down their car window, give you the finger and shout I hate you!"

But Johnston's career was soon over. "My sister was seriously ill and I had to go home to look after her."

Did he know what he would do once he retired? "Not a clue. I'd spent most of my wages on photographic equipment. That's my first love. I even had a studio in Liverpool. You see, I was never made to be a footballer mentally. I preferred to hang around with the press photographers after the games. They would use my darkroom and I would study their techniques."

After retirement, he got into television production, using his photography skills to help him as he worked for Kerry Packer and Rupert Murdoch. He made a packet by devising a globally successful TV game show concept, "The Main Event".

And then he made some more money from clever inventions. The one you probably haven't heard of is "The Butler", a piece of mini-bar software. "As you can imagine I spent a lot of time in hotels. Honour bars are one of the biggest drains on hotel revenue. So I surrounded each item in the bar with sensors so that if you took a bottle of beer out it sent a signal to the room's telephone and it would appear on your bill. Simple really."

He patented the design and now says between 15,000 and 20,000 Butlers have been sold. "I've still great hopes of it being a global success," he says.

But his most successful invention has been the Predator boot. He may have designed it with a children's market in mind, but as soon as it was licensed by adidas and became the boot of choice of David Beckham, Zinedine Zidane and Jonny Wilkinson it became much more expensive. It now retails at £120.

The Predator is essentially a network of cunningly positioned rubber and carbon-fibre nodules on the surface of the boot. When Beckham bent the ball into the proverbial onion bag against Greece, or when Wilkinson lofted the ball winningly over the heads of despairing Aussies ("Damn him! The bastard!" says Johnston with proper Aussie pride), they hit, says Johnston, a sweet spot. The knobbly rubber nodules of the Predator increase the surface area of the sweet spot, making it more likely that the target will be hit. "It's all about reducing the margin for error and improving control by 2 or 3 per cent."

Reebok and Nike weren't impressed, but adidas expressed an interest. "They did say, though, that they wouldn't accept the Predator on my recommendation because they had never heard of me as a footballer. So I went to Munich and got Franz Beckenbauer and Karl-Heinz Rummenigge - who both wore adidas boots - and I filmed them trying the boots out and saying the Predator improved their performance."

Adidas then snapped it up and the boots became the must-have of football's aristocracy (or at least those who don't wear Nike).

Johnston has just patented another boot design called the Pig, which, like galoshes, straps on to your boots, again aim to improve footballing performance. It's called the Pig because of the squealing noises it made when Johnston tried it out in the wet. He wants the Pig to retail for £20 - so impecunious kids can take advantage of it - and is keen to find out if any venture capitalists will help him develop the product.

But, first of all, Johnston is in competition to win the £25,000 Designer of the Year award, a contest parallel to the Turner Prize and aimed at showcasing British design talent (Johnston has lived in England for nearly a quarter of a century). A winner will be chosen from the shortlist of four in May.

"I don't think I've got any chance of winning," says Johnston with charming modesty, "but it doesn't really matter. To be shortlisted is the real honour."