A world record auction price for a violin was set at Christie's in London last month. The Stradivarius dated 1727 fetched £947,500. But how much is that fiddle worth that your father used to play? The first warning is: don't trust the label.

Mr Jonathan Stone, musical instruments specialist at Christie's, says: "A lot of instruments have labels inside them which purport to be those of the maker but they're not. Frankly the label is often misleading or fictitious."

But don't despise that neglected violin. It could be valuable. Mr Stone says: "There's a lot that can be done technically to alter the way a violin sounds. So a battered violin from the attic is not necessarily valueless. A cheap commercial violin made in China is not going to sound as well as a great Italian instrument. Violins from certain schools or makers [no matter how battered they may be] have the potential to sound well."

The maker, model, the date, its rarity, who has owned it, evidence to authenticate its attribution and its sound are features which affect a violin's value, according to Mr Stone. Despite the recent world record at auction, instruments have been sold privately for even more.

But if the chances of finding a lost Stradivarius in somebody's attic are, as he put it, "infinitesimal" some quite valuable instruments or bows do turn up.

Mr Graham Wells, director of the musical instruments department at Sotheby's in London, confirms that a "terrifying percentage" of fiddles have fictitious labels. "You would never believe how many there are. We see about one a day. The problem is you have to check them all out. One might be the real thing."

He explains that German factory fiddles manufactured mainly between 1850 and 1900 were "not made to deceive" even though they have labels which are "exact copies of the real thing". It was standard practice at the time.

While deliberate fakes have been made since the 17th century, honest makers simply modelled their instruments on successful ones. It was a safer commercial bet to copy than to re-invent. His advice is: "Don't rely on the label at all. Most really top instruments by the big names will have certificates of authenticity written by the dealer." But he adds that the certificates of only a small number of dealers have credibility.

In Ireland the premier maker was Thomas Perry in Anglesea Street in Dublin and died in 1818. He too has been faked in Germany. "At least a third of the Perrys I see are German factory fiddles," says Mr Wells.

Age alone does not dictate value. "There are extremely expensive 20th century violins and extremely cheap 17th century violins. Likewise, while Italian instruments are the premium ones, some Italian violins are of poor quality."

Mr Wells says violins can fetch "an enormous spread of prices". An ordinary German commercial copy can be worth between £100 and £200. If it's neglected and has fittings missing it can cost £100 or more to fix. "This assumes there are no cracks. But if it has cracks it really has no value at all."

Bows can also be worth a lot. Sometimes a case has a valueless violin but the bow could be worth £1,000. "You can spend up to £10,000 on a bow. The record price at auction is £72,000."

Mr Wells says that with a valuable instrument "when you retire you have a splendid nest-egg. Spending money on a good violin has always proved to be a wise investment". The recent Christie's sale would seem to confirm this. That Stradivarius which fetched almost £1 million last month was purchased in 1958 for $24,000.