Two years ago they sold their car, believing they could get along quite happily without it. They have persevered and today they both manage to commute thousands of miles a year, sometimes to remote destinations, relying on trains, buses and these odd-looking Brompton bicycles.
Looking a bit like an exercise bike on wheels, the Brompton folding bicycle does leave the rider somewhat open to ridicule. But the Henshaws have learnt to cope.
"Children are the worst," says David. "They can't pass you without making a comment. We get 'Why's that man on a child's bike, mummy?' or 'Like your bike, mister!' Or gangs of little boys on mountain bikes follow you up the road. You have to have a ready riposte."
Undeterred, David and his wife believe the folding bike is a potential saviour of the environment, the green answer to our traffic-clogged cities. And they have made it their mission to spread the word.
"We want to promote a form of travel that's not widely understood and can solve transport problems in this country," says David, aged 35. "That's how we see it - we have a message to put across.
"It's an option that people don't realise is possible. We take these bikes round the supermarket with us. We don't have to worry about parking. We go into central London to visit friends in a flat. You just fold the bike up and take it up the stairs."
In October 1993 they founded the Folding Society, a club for folding- bike enthusiasts everywhere. Membership costs pounds 4 a year and it now has more than 300 members, with some in Australia, the US and Germany. A recent enthusiastic recruit was Sir Clive Sinclair, himself not averse to unusual forms of transport.
David and Jane's terrace house on the edge of Castle Cary has panoramic views of Glastonbury Tor 12 miles away. Their dining-room has become a chaotic office cluttered with paper and bicycle parts, where they produce their bi-monthly magazine, The Folder, on an old PC and an Applemac.
You really do have to be an enthusiast to appreciate The Folder. In its pages David and his brother Peter report on road-testing the latest Brompton, Bickerton or Moulton bike; members swap useful information (such as where to get grease to stop your pawl springs rusting); there is even a regular column called Foldomania. In one issue, the column's author attempts to describe with a cut-out diagram the precise workings of his folding caravan.
David Henshaw used to run a photocopying business and Jane, 28, was a nanny. But when the business failed they turned to "animal aunting" - staying in other people's houses to look after their cats and dogs. Now his passion for the folding bike has landed him a part-time job with the manufacturer Brompton Bicycle Ltd.
"They liked our magazine so much that they bought the founder," he quips. "I just go around dealers spreading the word about the Brompton folding bike, explaining how to fold it, getting dealers to explain to people how it can change their lifestyles.
"I've even got a mobile phone and a company bicycle. Several shops have told me I'm the only representative who actually arrives on the bike and they like that. A lot of bike reps are rather tubby, besuited people who arrive in their motorcars. That really isn't very helpful to the image."
Standard charge for taking an ordinary bicycle on a train is pounds 3 per journey and some trains will not carry bikes at all. But if the bike is fully folded it counts as luggage. David's Brompton folds down to 2ft square by 10 inches across and weighs 25lb. A seat pulled out here, a wheel clicked into place there and within seconds you can be off pedalling.
Fellow commuters usually react with a combination of curiosity and admiration. One middle-aged passenger asked David for a go recently and went wobbling off across the station platform, to suppressed giggles from teenage onlookers.
Jane says: "If you're sitting on the station with it folded up, people come along and say 'Ooh - what's that?'. You usually have to give them a demonstration. I did one on the train once with the whole carriage watching."
But despite the novelty value, David firmly believes in the folding bike. And if the success of Brompton Bicycle Ltd is anything to go by, so do many others. The company started making Bromptons in 1987. Now it employs 16 people at a factory in Chiswick, London, producing 100 bicycles a week, and this year it won a Queen's Award for Exports. But the bikes do not come cheap. Prices range from pounds 364 to just over pounds 600 for top-of-the-range models.
"Apart from trains and buses, these funny little machines have become our sole means of transport," says David. "While pet-sitting we've never had to turn a job down, though I certainly expected to.
"Looking at the map you see big holes between stations; you think good grief, I couldn't get there. Very occasionally there's a bus involved. But it's nearly all done by train and then a cycle ride typically of five to six miles - anything up to 12."
Surely there must be times when they long for a nice warm car, perhaps when slogging uphill through torrential rain?
"No," says Jane. "Once you've got rid of the car you haven't got that temptation. And because you've not got the cost of running it, if you really need a taxi you can afford it. But we've never had to resort to it.
"Some friends think we're slightly strange to use bikes all the time. People seem to have a blind spot. They ask 'Where's your car?'. We explain that we haven't got a car - we don't need one."
They believe the word is spreading. They even managed to convert their lodger into a folding-bike devotee. "The message is getting around," says David. "Not all our members use folding bikes for green purposes, but a tremendous number do. For instance, taking it in the car to the outskirts of town, parking and then cycling in. People are learning about these things."
One Folding Society member recently had a set-to with the National Trust, claiming it turned her down for a job because she refused to buy a car, even though she did not believe the job required one.
David Henshaw wants employers to have a more welcoming attitude to bicycles. "Companies paying for parking spaces in the inner city could save themselves an awful lot of money. If you're riding a folding bike to work, all you need is a locker to keep it in. That has to be worth considering."
Brompton Bicycle Ltd, 2 Bollo Lane, Chiswick Park, London W4 5LE. For information on stockists call 0181-742 8251Reuse content