How can you tell if an item of garden statuary is valuable or only fit for the skip? According to Mr James Rylands, a consultant in garden statuary with Sotheby's: "Anything associated with water is always popular - any sort of ornament. Because you can see it, it moves, you can hear it, it's got so many more attributes than just a bench or a statue."

For instance, a limestone wall fountain recently sold at Sotheby's for £5,750 sterling (€8,780). According to Mr Rylands it had "basically been made-up. Someone had taken a sort of trough and put it on the front, so you weren't buying rarity there. You were buying a piece of decoration".

But if a water feature is of a scale that it could be fitted in a conservatory, it usually has a premium on it, which was just about the case with that limestone wall fountain: "It was about six foot high - you could actually have that going in a conservatory. So you're beginning to edge towards an indoor price, which is another consideration."

At the same auction in May, a rare and impressive terracotta fountain by the Glaswegian company Garnkirk, dating from around 1870, sold for £106,000 sterling, the most expensive object ever sold in Sotheby's sales of garden statuary.

Bronze objects can be more valuable if they are patinated, that is, if they have "gone green", he says. If it's a stone object, it should fetch more if it's weathered and looks old. But this is not the case if the object is of white marble, which loses value if it is overly weathered and becomes "sugary".

So, generally, looking old helps. But not everything that looks old is old. Some things may be only a few years old but if they've been under trees or made of a certain stone they may weather very quickly and look hundreds of years old. Says Mr Rylands: "In short, you pay for the moss."

To know the age of an object, you've got to analyse what it's made of. If it's made of Portland stone, a hard limestone, it weathers slowly. But if it's made of one of the Italian limestones like Vicenza stone, a very soft limestone, it will age more quickly. "What would take Portland a hundred years to weather, you can achieve in five years with Vicenza," he says.

Kilkenny marble, which is hard and dark, is much sought after and, indeed, Irish sculptures carry a premium, he says. Kilkenny is "almost like a black fossil but really hard. And on some of them the quality of carving is fantastic bearing in mind how hard the material is". The last "really decent one" Sotheby's sold was some 10 years ago. It was a really "beaten-up cherub" missing an arm and his bow "but what was really good about him was that because Kilkenny marble is so hard, it weathers, it patinates in a really wonderful fashion. It weathered and had this wonderful pattern or moss and lichen and things like that. I think it fetched about £7,500," he says.

Cast-iron seats by the Coalbrookdale foundry, produced in large numbers in the latter half of the 19th century, vary in price from about £800 for one of the more common patterns like fern and blackberry up to say £10,000 for seats in the passionflower pattern.

But there are fake Coalbrookdale seats about, made by taking a mould from an original and immersing the copies in sea water to age them. A tell-tale sign is that the new ones tend to have quite visible casting marks on the seams, which haven't been properly filed off and finished.