Original props and items associated with a production are always the best find for a collector, but these are often hard to locate in television.
Unlike films, which have a higher profile and often a bigger budget, many of the props and wardrobe items from popular shows of the past get recycled for other programmes or simply thrown away.
Look out for items associated with cult series such as The Man From Uncle, Dr Who, The Professionals, The Sweeney, The Avengers and Thunderbirds. "Anything from Gerry Anderson is collectable - original puppets can sell for £30,000 or more," adds Cowdry.
"One factor that helps improve collectability is if a TV company starts repeating shows, which then generates interest again," he says. "Three-year-olds today are into Thunderbirds, even though the original series went out in the Sixties."
On this basis, Cowdry has a tip. "The Persuaders, which originally came out in 1969, might well prove to be a goldmine," he suggests. "There was only one series produced and now there is the movie with Ben Stiller and Steve Coogan, so interest will pick up again." He also has tips for up-and-coming collectables. "Only Fools and Horses is something which is bubbling and although collecting hasn't really taken off yet, it will," Cowdry adds. "I'd also go for anything that Ricky Gervais laid his hands on from the set of Extras or The Office as these would probably be a good investment."
At the popular end of the TV memorabilia market, toys are also an interesting play. "Every single year we think it is going to flatten out but TV-related toys are becoming more and more popular," says Andrew Reed, a cataloguer at Vectis Auctions.
"People who watched a show on TV can relate to the toy - plus there is a wide range of collectors, from people who remember programmes from the 1940s right through to those keen on the modern day. Even modern toys you can buy in very low limited editions will double their money in six months to a year."
There are two sides to this market. A lot of younger people just want an example which might have been played with, but the slightly older collector in their thirties and forties wants one in mint condition with its box.
"We are getting so many people like solicitors and bankers who are investing money into toys because they don't want to put it into the stock market that it's pushing up the prices," explains Andrew. "The lower end of toys between £50 and £100 is getting pushed up because the bigger stuff is being pushed out of reach of the standard person because of these big investors."
Hugo Marsh, from the auctioneers Christie's, reports a similar level of interest. "Across the toy world anything associated with TV has a life of its own," Marsh says.
"Dinky's Thunderbird Two - the green, pod-like craft - would five years ago have fetched £70 to £80 but it is now reaching about £150. Doctor Who was surprisingly poorly exploited in 1963 - the BBC licensed a Louis Marx battery-operated Dalek in 1964 which was made in Hong Kong and would now fetch £150, but beware as they were reproduced in the 1990s."
If you kept your childhood toys it is certainly worth checking what you have. "There's a lot of toys from the late Seventies that people have in the attic which they don't realise are worth money," says Andrew Reed.
"Even toys that are played with, such as He-Man, are wanted by collectors because there isn't a lot that's come onto the market. Eighties TV programmes such as The A-Team and Magnum, PI are also coming into it." Even so, for investment purposes - rather than pure nostalgia - quality and condition is crucial.
Spotting the next cult TV series is always a challenge, and don't forget American TV. Anything from The Sopranos is a good bet now the series is finishing its run. Similarly, M*A*S*H was one of the longest running shows and its memorabilia is still selling well.
Also look out for British TV shows that have also been popular in the US. Fawlty Towers, for example, is collected both here and in America.