You may well be suffering delusions of grandeur about that pocket watch, owned by your great-grandfather and carefully handed down through the generations.

"People might think their watch is worth a lot of money, but it's just one of many millions that were produced," says Mr Oliver Saunders, head of the clock and watch department at Bonhams in London. It's no exaggeration to say that pocket watches were "produced in literal millions". He says some US firms produced as many as 50 million watches in the 19th century.

Pocket watches were made from around the 1650s. Wristwatches came out about 1914. Most pocket watches, being in silver cases, can be easily dated by the hallmark, Mr Saunders says.

Watches have to be "something special to be of any great value. You could buy a silver pocket watch for probably £10 or £20," he says. Even watches from the 1820s like a "farmer's verge" can be bought for "about £150". If they have a plough or train on the front, "that will push the value up to £200 or £300".

Mr Saunders says that because there were so many of them, the collector can be quite choosy. Collectors tend to go for a well-known maker or they choose a watch with something unusual about it such as a striking mechanism like a repeater or a chronograph, a mechanism within the watch that allows you to start and stop time.

Collectors are "mostly interested in the earlier watches, say from the 1700s, when watches were made in very small numbers".

A sale of fine clocks and watches at Bonhams on April 14th has a very early silver-cased watch with an alarm, dating from about 1680. Mr Saunders says: "It's quite unusual. You don't get many alarm watches. It's 55 millimetres in diameter, which is quite a large watch, with a silver dial." (Estimate: stg£400-£600.)

A "coach watch" dates from circa 1670. "That's huge. It's 95 millimetres, a beautiful museum-type, very rare piece. It's rather like a very large portable timepiece" with an alarm, rather than designed to put in your pocket, he says. (Estimate stg£10,000-£15,000.)

A very fine platinum 1930s Breguet pocket watch with a perpetual calendar and chronograph is estimated at stg£15,000 to £20,000. It's technically interesting because it has a perpetual calendar: it anticipates leap years and months of unequal length. A triangular 1920s Masonic watch with various Masonic symbols is a "very novel, completely bizarre-looking" watch. (Estimate: stg£600 to £800).

Mr Ken McDonagh, proprietor of Dawson Jewellers the Watch Gallery in Dublin says watches are a very specialised area and a lot of jewellers and antique dealers tend to steer clear of them. He says "very few are doing pre-owned watches" in Ireland but that there's a "huge market in preowned quality watches". His firm is specialises in repairing watches of any age.

For free estimates of the cost of repairing watches, contact Mr Ken McDonagh of Dawson Jewellers, Dublin. Telephone: 01 662 4437.

For further information about the Bonhams watch sale, contact Bonhams' clock and watch department. Telephone: 00 44 171 393 3931.

Ms Rachel Coleman, jewellery specialist from Bonhams' Knightsbridge Galleries, in association with James Adam Fine Art Auctioneers, will offer free advice and auction valuations on any item of jewellery in Dublin on April 27th and 28th. For further information, contact Ms Alison Moore, 00 44 171 393 3970.

The next Dublin toy and train fair will take place in the Rochestown Lodge Hotel, Rochestown Avenue, Dun Laoghaire, from 11 a.m. to 6 p.m. on Sunday, April 25th.