While pupils bowed their heads and the quiet murmur of assembly prayers filled the school hall, Les Turner would slowly jog on the spot. The head of St Patrick’s Roman Catholic primary, in Lancashire, had eight pounds to lose in two weeks, so he used every spare moment to fit in some exercise.

At the start of this year, he weighed 17-and-a-half stone. Now he is in training to complete the Great North Run half-marathon in October.

The transformation began when the 55-year-old pledged to lose three stone in two months as part of a makeover series filmed by Granada TV.

But he quickly realised there was no need to hire a professional fitness coach. Instead, he garnered encouragement from pupils at the Morecambe primary.

“I would arrive at school at 7.30am and do laps around the playground,” he said. “When the children came in, they would do 10 laps with me, then another set of children would take over. They stopped bringing me biscuits on their birthdays. When I had only two weeks left to go, I used to jog through assembly prayers. It helps to be mad.”

When his will-power began to flag, the thought of having to admit his failure to the school at assembly was enough to keep him on the wagon, he said.

Mr Turner reached his target weight with two days to spare, then went to Manchester for a makeover supervised by presenter Derek Hatton, the former deputy leader of Liverpool city council famous for his sharp suits and sending out redundancy notices to council staff in taxis in 1985. His hair was dyed auburn and given a spiky cut. The beard came off and beauticians shaped his eyebrows and rubbed haemorrhoid cream around his eyes to smooth over wrinkles. Next came the clothes shopping.

“They put me in a velvet shirt with flowers in brown, red, grey and all shades in between,” he said. “I’d never have bought it in a million years.

You think, ‘Wow, that shirt is loud.’ I used to buy jeans at Tesco - tubbies can’t be choosers.”

Back at school, his new look earned a sharp intake of breath at assembly.

And since the programme was filmed, Mr Turner has lost a further one-and-a-half stone, and his waist has shrunk from 40 to 34 inches.

He said: “In 1969, I used to shower in my jeans to get them tight. The girls at university voted me rear of the year. Now, I’ve bought tight jeans for the first time again. My two sons are in their 20s, and I’ve started borrowing their clothes. I’ve decided to grow old disgracefully.”

His sons are not the only ones to find that there is a downside to his new image.

“Losing weight has helped at school, because I’m fitter and I have more energy,” he said. “It’s given me a whole new lease of life. It’s quite tiring for the teachers. I think some of them wish they had the fat old sod back again.”

* adi.bloom@tes.co.uk