The North Ronaldsay flock, which lives at Northamptonshire Wildlife Trust's High Wood, near Upper Stowe, graze on acid grassland, a rare kind of soil which is almost unique to the UK.

Thanks to an £18,000 grant, maintenance of the land, and the welfare of the sheep, is now secure for several more years.

The money has been donated by the Veolia Environmental Trust, which gives out grants to green projects using money generated through landfill tax.

Cathy Wainwright, grazing manager for the Wildlife Trust, said the sheep were extremely rare.

She explained: "They are one of the Northern European primary breeds which are unimproved.

"They haven't been specially bred. This makes them ideally suited for this environment as they need to fend for themselves.

"The money we have been given will go towards further improving the biodiversity of the site, checking on the sheep and training volunteers."

Fred Key, a Wildlife Trust volunteer, said: "Most of Northamptonshire is neutral grassland but some areas, which are higher up and haven't been tampered with, remain acid grassland.

"The soil is more acidic, which allows plants like heather to grow on them. Plus, all the acid grass sites we have found are sites of special scientific interest."

Ossie Dodds, board member for the Veolia Environmental Trust said: "There are very few acid grasslands in this area and it seemed to us it was worth preserving and protecting."

Anyone interested in finding out more about acid grassland or conservation grazing can log on to www.wildlifebcnp.org