Before you chuck out your old gardening tools or that urn you inherited from your grandmother, check out their value because Jonty Hearnden, presenter of the BBC1 show Cash In The Attic, says you could be sitting on a goldmine. He reckons you may be able to sell anything from old paving slabs to large concrete urns, ancient tools, aged weathervanes and benches for hundreds - or even thousands - of pounds.

People who've inherited property or are clearing houses for relatives are the most likely candidates to find an Aladdin's cave of treasure in the garden.

"The most common items that may be lying around are garden urns or statuary of the 20th century, the moulded urns and the moulded figurines and birdbaths which you think are just old and don't have any value," he says.

"Actually, there's a very good second hand value for those items because dealers and interior designers like urns which look really weathered. They might only be 20 to 40-years-old, but there's a definite market for antique-looking reproduction garden urns and other items."

Such items may be made of concrete, otherwise known as reconstituted stone, and the ones fetching the most money have an 18th or 19th century feel to them, he says.

"I went into a shop the other day where there were four identical urns that were 2ft tall by 1ft wide with box bushes in them. They looked a million dollars and they were probably around £400 each."

Birdbaths, weathervanes and other ephemera all have a value, he says.

"You could easily get £100 for a birdbath in an auction sale. It has to be weathered so that it doesn't look new, which can take a few years."

Coalbrookdale garden benches, which are made of highly ornate and Victorian-looking cast iron, are extremely sought-after items which can fetch between £3,000-£6,000, he observes. The 19th century benches are often stamped Coalbrookdale or C-B Dale Co, and the seat is probably made of timber slats.

"People may be sitting on a fortune because they just don't realise how expensive these benches are," he says.

There is also a market for old garden tools, desired for their craftsmanship.

"Garden tools have a certain value. Don't just throw them away. There will be somebody out there who wants them because they just love the feel of old wooden handles and cast metalware that's not made to the same standard today.

"Good places to sell those sorts of objects are eBay and car boot sales, as people will often pay between £10-£30 for old, good quality forks and trowels if they look presentable. Expect to get less on eBay. Go for the car boot sale if you're selling anything under £20."

Also, in a house that is Victorian or older, don't forget to look down, people will pay good money for garden edging tiles from the 19th century.

"The Victorians produced a lot of garden design materials. It was the era of the municipal space where you would have a garden park which was manicured, with flowerbeds, run by the community or local town council. They were very big on gardens."

Undamaged marble is especially lucrative and will fetch thousands, while urns and ornaments can make the top hundreds.

"Where I find people need a lot of guidance is when they've inherited properties or are dealing with deceased estates. That's when people go into the garden and find hidden treasures."

Now, a new website, cashintheattic.com, has been launched to help people value their items. After uploading a few snaps of items to the website, experts will value their worth, charging from £5 per valuation. The site can help visitors research pricing and provenance.

Spring and summer are the perfect times to sell garden memorabilia, Hearnden adds. Dealers and salvage companies will buy many gardening items, while there are auction houses such as Summers Place Auctions in West Sussex, which deal specifically in high end garden statuary.

Frost damage will inevitably devalue items which would otherwise fetch a good price, he agrees.

"Once ornaments are damaged by frost, they won't have value. Check for hairline cracks because dealers won't want to buy items with hairline cracks."

Even paving slabs may be worth selling, he observes.

"York paving stone, initially laid municipally for street paving, has all now been taken up and put down into people's kitchens and interiors or put on people's patios because they really love the look.

"Now, companies make reproduction York paving slabs because it's so desirable. If you have York stone as a patio you don't want, there's probably a buyer for it. There's usually a salvage man who will want to pay you good money for it and take it away for you, all at the same time."

Even garden gnomes may fetch a price in the future, he adds. Like the William and Kate gnomes at this year's RHS Chelsea Flower Show.

"Those will be worth an absolute fortune one day, simply because they were the first garden gnomes allowed in to the Chelsea Flower Show and due to the subject matter and the fact that she's pregnant with the next in line to the throne, boy or girl."