Shepherds watch their flocks by night as rustlers lie in wait

It wasn't a night to turn a dog out. The horizontal sleet turned to snow as we drove higher up the moor and it seemed hardly likely there would be any rustlers out either.

Tony Flintoft disagreed. Peering into the murk through the windscreen of an ageing Range Rover, the farmer grumbled at how the snow and mist provided good cover. With a sheep fetching up to pounds 80 on the black market, "the buggers'll be out somewhere".

Sheep rustling is an expensive menace to the livelihoods of farmers across the fells of northern England and the Scottish Borders. But a small group on on the North York moors believe they have checked the rustlers in the national park by nightly "Sheep Watch" patrols.

More than 3,000 sheep were stolen off the moors in 1995. But so far this year, according to data collected by Sheep Watch, only 208 have been taken. "People know we're about and it's gone a lot quieter - for now," said Mr Flintoft, who started Sheep Watch two years ago after losing 133 from his hill farm at Snilesworth.

He is getting reports from the Yorkshire Dales and the Borders of a corresponding increase in rustling there. North Yorkshire police statistics seem to bear out this shift in tactics. Their records show 1,800 sheep worth pounds 77,000 stolen this year, with the biggest round-ups in Swaledale.

The "mad cow" crisis sent sheep prices soaring. Quality animals which fetched pounds 50 at auction a year ago are now going for around pounds 100.

The loss to the farmer of a young breeding ewe is potentially three times as much. And replacements may wander. There are no fences or walls on the heather moorland but the black-faced sheep have a generational instinct for their "heft" - or home ground.

They are rustled in small vans and cars. Ten could be crammed in the back of a Ford Escort van or three or four hobbled - legs tied together - and put in a car boot. "There are no animal welfare considerations in this game," Mr Flintoft said.

Quality breeding animals could be sold on at an auction well away from the moors. Coloured dyes can be washed out of the fleeces and tags removed. And once the sheep is sold at auction, the aggrieved farmer has no legal claim on it even if he can trace it.

The police say there is no obvious "sheep trail". But most of the animals are probably slaughtered on unlicensed premises and a lot will end up on plates in restaurants.

The rustlers, though, know the unmarked tracks of the moors as well as the patrollers. If anyone is caught in the act, they may be known to one of the eight watchers who have a shepherd's intimate knowledge of the tops and valleys.

Using five vehicles in radio contact, the group monitors the lonely moorland crossings, logging numbers and watching for vehicles "not sitting right". Suspicions are reported to the police. "We're not vigilantes," Mr Flintoft insisted.

The National Farmers Union's Mutual Insurance company has figures showing that rustling - of pigs and cattle as well as sheep - costs UK farmers pounds 3.5m, but it suspects the true figure is much higher.

Sheep Watch says moor-men have traditionally been loath to report thefts, believing it reflects badly on their shepherding skills. When Mr Flintoft and Brian Cook, who had had 40 sheep taken from his Bilsdale farm, set up the group there was a good deal of derisive comment.

"Rustling has been going on for donkey's years and farmers have put up with sheep going in ones and twos," Mr Cook said. "But when pounds 2,000 worth goes at a stroke it's a lot of money and I thought it was time we did something about it."

Parked up at 2am in Hutton-le-Hole with the sleet battering his 4x4, Norman Tinsley of Farndale would rather be at home but believes his business is at stake. He has lost nearly 200 sheep over three years.

"Do I come out for maybe four hours three or four nights a week and reduce the risk of theft by 60 or 70 per cent, or do I go to bed thinking `Are the buggers about again tonight?'"

Last week Mr Tinsley had his answer. Gathering in his flock, he cast an eye over the 600 ewes for familiar animals and they were all there. "It is marvellous to gather sheep for once and have the figures tally."