He became a household name for his portrayal of Boycie, in Only Fools and Horses. It became the nation's favourite sitcom of all time and attracted more than 24 million viewers – more than a third of the population – for a single episode.
But away from the screen, Mr Challis lives a quiet life on the Shropshire and Herefordshire border with his wife Carol. They live at Wigmore Abbey, an ancient mansion that they bought in 1998.
And Mr Challis has recounted his arrival at the property in a new book: Wigmore Abbey, The Treasure Of Mortimer.
He tells how he and arrived there in the late '90s, throwing themselves into the task of restoring the abbot's lodging – the only habitable part remaining of what had once been a great monastery.
They always had an interest in gardening and once they'd got on top of the house restoration, they went out to have a proper look at the land around them, where they uncovered extensive gardens that were badly neglected and overgrown.
John and Carol soon found that they had to strip everything back, and start again with a more or less clean palette on which to create their new garden.
Their activity was interrupted when the BBC proposed that Wigmore Abbey would be the perfect location for the Only Fools & Horses spin-off, The Green Green Grass.
Delighted with the prospect of not having to drive to work, John and his former OFAH co-star Sue Holderness quickly recreated Boycie and Marlene as the out of place country 'squires' in a show that ran for four series.
Since then, John and Carol have persevered with the task of recreating the gardens, reaching a point where visitors have come from far afield to see it. It was even chosen to be featured on one of Alan Titchmarsh's Chelsea Flower Show programmes.
Wigmore Abbey, The Treasure of Mortimer is a charming, personal book, lavishly illustrated with photos of the interior and the gardens through the seasons.
Mr Challis said he had been living in London before his big move: "We knew already that it was far too much to take on, too far from London, too old, too dilapidated, but somehow, it seemed we were being pulled towards it by an unknown force.
"We drove straight to Hereford, found the agents and made an appointment. The next morning we turned up at the house, this time driving down the gloomy drive with an entirely new sense of adventure, as if we'd set out on a journey into the unknown.
"The owner Mr Rostron ushered us in and suggested we had a look around on our own. The building was even more extraordinary inside than out. As we wandered, we sporadically gasped with surprise and excitement at yet another amazing room or feature.
"We were standing in a stone-flagged corridor, looking past an ancient oak staircase, through a door into a bare-walled, high-ceilinged chamber. We walked closer. A shaft of light from a south-facing tracery window cast a gleam on a brass crucifix standing on a rugged table. At that moment, wisps of some ethereal entity seemed to be seeping up through the floor and swirling round the sunlit crucifix.
"This must be some kind of sign of divine providence, I thought, although I kept the thought from Carol, who is less tolerant of this kind of fantasising and in this case she would have been right; the owner of the house had just lit a fire in the undercroft beneath the parlour, no doubt to enhance its attractions when we came to inspect it."
Mr Challis added that the clincher came when his wife realised she had an ancestral connection with the property. Her relatives had previously lived at Wigmore centuries ago – though she had had no idea.
"Later, surrounded by the mellowed oak panels of the library, we came upon another potent sign from the spirit of the abbey. On the wall above a desk was a framed depiction of a coat of arms topped by a clenched fist holding a red anchor.
"Carol did a double take; she was sure she recognised the arms, which were also displayed on the walls of St Mary's Church at Cerne Abbas in Dorset. They were the arms of the Cockeram family, Carol's ancestors through her maternal grandmother's line.
"It turned out that in 1556 Philip Cockeram of Wigmore had been granted arms by Queen Mary as well as, in consideration of the sum of £309.4.0, the old abbot's lodgings and grange, some 18 years after the monastery at Wigmore had been dissolved in 1538."
Mr and Mrs Challis decided to move to the area. "When we arrived at Wigmore Abbey on 23 July 1998, we both knew we'd entered a critical new phase of our lives. We welcomed it and were committed to it, although I found it quite disorienting.
"Unlike Carol, I'd never lived in the proper country and wasn't prepared even for some of the more minor changes from urban life. For instance, it took me a long time to get used to the general absence of street lighting, even in some of the villages.
"And while I was used to a certain amount of birdsong in the leafy purlieus of East Sheen, the racket a feathered chorus can kick up of a summer's dawn among the spinneys and hedges of this deeply rustic corner of England came as a shock. I had to get used to the lack of traffic in the lane that passed our house."
He added: "We also knew that our lives had now changed fundamentally. We had to get used to a slower pace, where less happened but what did happen was somehow more vivid.
"Our lives were no longer Londoncentric; our everyday town was Ludlow, a fascinating and extraordinarily well-preserved medieval hill town. For bigger stuff we had to drive down the old Roman road to the compact, old fashioned city of Hereford, with a cattle market, a cathedral and a population of just 60,000. Some Metropolis!"
His work as an actor soon dried up with casting directors assuming he'd moved out of London to open a B&B. English Heritage also took a keen interest in the property because it was listed.
"I still had some useful stand-by voiceover work coming in, but with a six-hour round trip on the train to London, it was neither convenient nor particularly profitable. I began to think that as far as most casting directors were concerned, I'd moved too far beyond the pale, out of sight, out of mind – while the old safety net which Only Fools & Horses had provided for me over the previous nearly 20 years was gone.
"In moments of characteristic paranoia, I had bouts of thinking that in buying Wigmore and moving out here I'd made the stupidest mistake of my career. I found myself oscillating between periods of euphoria and bouts of terror – joy at just being in this beautiful, uncluttered part of the world, occupied with such an historic house, and fear of never working again or being able to pay for it all."
When Mr Challis approached his 60th birthday, his wife decided to throw a surprise party. She invited the cast of Only Fools and Horses, including Sir David Jason, and a happy afternoon led to the creation of a new TV show; The Green, Green Grass.
It focused on Boycie's move to the countryside and was filmed at Wigmore Abbey. Locals in Leintwardine and Ludlow were employed as extras.
On the morning of the party, Mr Challis was in the dark.
"I went to the top of the stairs. 'What the hell's going on?' I bellowed.
'What do you mean?' Carol's question floated innocently back up the stairs.
'What do you mean, what do I mean? There's a bloody great tent in the garden.'
'What tent?' 'What do you mean, what tent?'
'Oh, that tent! That's not a tent; it's an awning.'
'What's a bloody awning doing in the garden?'
'Stop shouting and get dressed. It's your birthday.'
It was indeed. "I was frankly overwhelmed and not helped in this by having downed several large gulps of festive wine to deal with the trauma of it all. I could see even more people arriving and queuing up to remind me how old I was and I became very emotional when most of the team from Only Fools & Horses appeared.
"Afterwards, through a woozy haze of gratitude, I saw John Sullivan, writer of Only Fools & Horses, approach. 'I've had a bit of an idea,' he said. 'I'll get back to you.'"
Two years later, Mr Sullivan pitched the idea of the Green Green Grass and it became a hit. The location fee helped Mr Challis to pay for repairs to his property.
The house has also featured on other shows.
"It must have been at least partially my public persona that prompted the BBC – never averse to promoting their own shows with a little cross-fertilisation – to get in touch in 2001 to arrange for The House Detective at Large and celebrated ruin-botherer, Dan Cruikshank, to visit Wigmore Abbey and subject it to his brand of forensic archaeology.
"I hadn't met Dan before, but he turned out to be a cheery soul, who behaved like a boy with a new toy as he wandered around among the ruins, picking up lumps of stone and reassembling them like blocks of stony Lego as Gothic arches on the lawn."
Mr Challis added that he and his wife were glad that they'd made the move.
"Carol and I have been here for more than 18 years now, and the house has become an enduring challenge for us. We have sought out and collected furniture and pictures from far and wide to endow the house with an eclectic mix of medieval and Mortimeralia to complement its extraordinary architectural features. Like many ancient buildings, every century has brought additions and changes, creating a charming mish-mash of styles, and the house is more remarkable for that."
Wigmore Abbey, The Treasure Of Mortimer, is available from Mr Challis's website, www.wigmorebooks.com, priced £30.
By Andy Richardson