This, perhaps, is why lime mortar can last for more than a thousand years: it is itself stone. Likewise a lime wash - lime putty mixed with water - painted on to a wall, forms a thin but durable white layer of limestone. With linseed oil or tallow added to the mix, it makes a waterproof but breathable coat, at a fraction of the cost of a modern vinyl emulsion. (Bennett warns: never use tallow for interior decoration as the stench on a hot day or in the heat of a fire is unbelievable.)

As befits such a versatile material, lime is staging a comeback. In 1988, Bennett's Lime Centre was selling one or two 40kg tubs of lime putty a month. Now it sells 15 tonnes. The restorers of Sutton Place, near Guildford in Surrey, have already taken 15 tonnes with as much more still to follow. Another customer is a national house-builder. "He usually builds little concrete boxes for the rest of us to live in," says Bennett. "But now he's building a huge new mansion for himself, and what do you think he's using? Lime. Nothing but lime mortar throughout."

One way for Bennett to spread the word is through his hands-on "Building with Lime" day courses, of which he holds about 25 a year at The Lime Centre and others at sites around the country. Most of his students are builders with an interest in historic buildings and their restoration. One such is Alan Brown, who runs a Bristol joinery shop, and has a "passionate interest in old buildings and old ways of doing things that I fiercely believe should not be lost". Others are amateurs, like Neil Bolton, owner of an old farmhouse in Suffolk, which hasn't been touched for 50 years, but which he is now restoring to perfection, using original materials.

Bennett began his buildings restoration consultancy 20 years ago when he used his experience as an irrigation engineer to devise a system for cleaning the west front of Wells Cathedral without damaging the limestone. "A two-week experiment turned into 5 years' work," he recalls. "The silly thing is that one is then seen as an expert, but I got a flush of interesting projects to work on."