At first glance, this has to be the worst holiday deal ever. No frills? This one doesn’t even have a lining. Accommodation is basic to say the least - bed is a bunk in a bare-walled dormitory - and the shower block is shared. As for the catering, it’s strictly DIY, as are all the chores.

To cap it all, you’ve got to spend your days doing real physical labour.

And here’s the best bit: they don’t pay you a single penny - you’re paying for the privilege.

So why, a few days into it, are the holidaymakers so cheerful as they gather under an oak tree to wash down their sandwiches with tepid tap-water from an assortment of plastic bottles?

The scenery here on the Long Mynd, the ridge of green hills that runs across the south of Shropshire, is every bit as beautiful as the brochure suggests. But don’t these people know they could be by a pool somewhere, sprawled on loungers rather than sitting on the supermarket carrier bags they now routinely use to shield themselves from nettles?

People who sign up for National Trust working holidays are used to questions like mine, and the group under the oak dispatch my incredulity with the same alacrity as they displayed when they began clearing bracken from the surrounding slopes at 9 o’clock this morning. “It makes a real change from doing the sort of things we normally do,” says a doctor. “And it’s a way to explore different parts of the country,” adds a librarian.

Everyone agrees that holidays like this are a good way to meet new people, and all have enjoyed working with professional wardens, learning how the countryside is managed and giving something back to the places that they love.

But it is Peter, an aeronautical engineer from Derby, who sums up the general mood. “The thing is,” he says, “it’s really good fun. Why else would people come back year after year?”

And come back they do. Ask the nine volunteer workers gathered on this valley side today how many have been on these holidays before, and six grubby hands shoot up. Some have repaired dry stone walls in the Peak District and others have rounded-up goats on the Isle of Wight. Between them, they have cleared rampant rhododendrons from the Cornish coast and helped stamp out the troublesome Himalayan balsam from Ullswater.

Ben, an IT consultant who, as leader of this group of volunteers, organises everything from grocery shopping to liaising with the wardens who set each day’s tasks, will never forget the week he spent at Belton Hall in Lincolnshire. It was while cleaning silver and tending the kitchen garden that he met Sarah, whom he plans to marry in the autumn.

So it’s no surprise at all that the three working holiday newbies in the group have already decided to sign up again next summer. Even their experience with half an acre of gorse, it seems, has not punctured their enthusiasm.

Anyone in the group will tell you about the gorse. In fact, if you have 10 minutes to spare, and you happen to be in Carding Mill Valley, a glorious spot on the edge of Church Stretton that has become one of the National Trust’s most successful education venues, they will even take you to see its withered remains.

It was the first working day of the holiday, and the group’s alloted task - clearing a firebreak around a timber house - had seemed simple enough. But gorse is nasty, prickly stuff, and this was a blisteringly hot day.

Together with the steepness of the site, it all added up to a gruelling initiation for people more used to wearing a shirt and tie and having conversations round the water cooler.

Needless to say, everyone slept extremely well that night, back at the brick and timber farmhouse that the Trust has restored and equipped as a base camp for its working holiday volunteers. In the days that followed, nothing was to match the gorse experience in terms of sheer physical exertion. Either that or, as their stamina built up, they became more used to the demands of the labour.

Constructing a wheelchair path alongside the tinkling river was a doddle after the gorse, they tell you. And as for the day they spent high on the moors of the Long Mynd, with only larks and eerily whirring gliders for company, building earth banks to keep motorists out of the heather - well, that really was a holiday.

And at the end of each day, as they cooked their communal dinner, or strolled out by torchlight, searching for the local pub that they knew, from the pile of Ordnance Survey maps in the lounge, must be just around the next bend, the legend of the gorse grew with the retelling. It would come up over late-night Scrabble, and among the groups who huddled over 1,000-piece jigsaws.

And here, as you help them make a medieval earthwork a fit place for archaeologists and dog walkers, they will say: “you should have been here when we cleared that gorse”. And as you take care to spare the clusters of foxgloves that add streaks of colour to what is already a picture-postcard view, you begin to think that, yes, perhaps you should have been with them on that field of battle.

For if there is one lesson to be learned from a working holiday - and other lessons might include “a change is a good as a rest”, “beauty spots need constant care and attention”, and “make sure there are no bees in the shower basin before you turn on the water” - it is that working hard for a common enterprise makes for camaraderie. And camaraderie is something that no amount of air miles can buy.

NATIONAL TRUST SUMMER ACTIVITIES

The National Trust is Britain’s largest charity, with 3.4 million members and 43,000 volunteers. If you’re looking for stuff to do with children over the holidays, there are 42 days of activities planned in the “Summer’s Sussed!” programme. You can download 10 regional wallcharts from www.nationaltrust.org.uksummersussed for details of activities happening around the country - from a Knights and Maidens day at Dunster Castle, Somerset to a 17th-century living history weekend at Chirk Castle in Wales.

FROM HABITATS TO HA-HAS - SO YOU WANT TO BE A VOLUNTEER?

It’s nearly 40 years since the National Trust first began organising working holidays as a way of involving the public in essential conservation work on its 240,000 hectares of land. In that time, close to 100,000 holidays have been enjoyed.

Prices currently range from pound;35 for a three-day January break spent clearing scrub from Nancy Astor’s estate at Cliveden in the Chilterns, to pound;240 for a week in August spent acting as a room steward and collecting litter at Lindisfarne Castle in Northumberland.

By and large, accommodation will be in a farmhouse or cottage converted into volunteer quarters, which means cooking facilities (meals are included in the price, but catering duties are generally shared) dormitories (bunks and sleeping bags) and shared toilet facilities.

However, those who feel that they put the trappings of communal living behind them once they left full-time education can opt for a premium holiday, which means the same amount of hard work in the day, but the comfort and privacy of twin rooms with en suite bathrooms and ready-made beds in the evening.

Types of working holiday also vary, from standard Acorn holidays for anyone aged 18 and upwards, to Oak holidays, where the minimum age is 40.

In recent years, the trust has also developed more specialised holidays that focus on particular types of activity. Wildtrack holidays, for example, are for biology students, amateur naturalists and lovers of wildflowers who want to get involved in survey work, and these include a day of instruction about the site (Orford Ness on the Suffolk coast, say, or Hardcastle Crags in Yorkshire) and its habitats.

There are gardening holidays in settings such as Baron Ferdinand de Rothschild’s mock French chateau at Waddesdon Manor in Buckinghamshire, and Wallington in Northumberland, where the gardens were designed with the help of Lancelot “Capability” Brown, who grew up in a nearby village.

And for those who prefer crowbars to secateurs, there are even construction holidays, where instructors teach the rudiments not just of dry-stone walling, but also building bridges (on the Lleyn Peninsula in Gwynedd) and repairing ha-has (at Kedleston Hall near Derby). Flip-flops? The chances are you won’t be needing them. But don’t leave home without a pair of steel-toed boots.

Details and booking: 0870 429 2429; www.nationaltrust.org.ukvolunteering