Demand for stone has hotted up to the point where dry stone wallers are using cement and farmers are locking their gates at night to discourage thieves.

There is plenty of stone, according to the wallers.

But the cost of it, especially when it has been shaped, comes as a shock to people who are not used to buying it.

David Balderstone, secretary of the West Yorkshire branch of the Dry Stone Walling Association, confirms that it is becoming fairly common practice to mortar the top course of a dry-built wall.

"The tops are often taken because they often have to be shaped, and a nice set of top stones, all the same height, is worth up to £8 a foot in old money – £30 a metre, " he said.

"With the amount of activity in wall repair now added to the health and safety consideration, a bit of mortar often seems like a sensible precaution."

Dairy farmer Colin Grice, at Westerdale, near Whitby, lost £500 worth of sandstone ridge and edging pieces off the roof of an 18th Century barn in one of his fields last summer – 70 metres from the nearest road.

He said: "They hadn't taken a vehicle into the field so they had gone to some trouble.

"My guess was it was somebody looking for the materials to finish a job, but I never heard any more about it after telling the police what I could.

"To cap it all, my insurer would not pay out, because theft of part of a building was not covered, although malicious damage would be."

NFU Mutual was not the insurance company concerned, but a spokesman for NFU Mutual, Tim Price, said insurance for commercial buildings separate from a main residence might well be conditional.

"You have to look at the exclusions you are agreeing to and decide whether it is worth paying more for fuller cover, " he advised.

He said professional criminals were targeting farm machinery, which could be exported and resold for many thousands of plants in eastern Europe, Africa and the Middle East.

His company is offering discounts on premiums in return for the use of immobilisers and electronic tagging.

In comparison, the black market in stolen building materials was comparable to rustling of sheep for the freezer – a relatively minor problem which went up and down with unemployment.

A farm insurance agent in Yorkshire agreed.

He said: "You can measure the state of the economy by the number of padlocks you see on farm gates."

Nick Coldbeck, a full-time waller based at Scarborough, said there was no need to steal stone.

He would usually be using fallen stone but could get anything he needed, including elaborately cut pieces, through suppliers like Dring Bros of Pickering and Fenby Stone near Robin Hood's Bay.

But even a load of simple walling stone is likely to cost £100 a tonne delivered and a four-foot wall will use two tonnes a metre.

He said: "My impression is that the main problem is people wanting stone for their rockeries.