New names are joining better known designers such as Arne Jacobsen, Alvar Aalto, Marcel Breuer and Charles Eames, which have sold in top auction houses for the past decade. "The design market was underdeveloped 10 years ago, but there has been a significant increase in interest in this type of furniture, particularly in the past three to four years," says Bonhams' Bruce Addison. "Ply in particular has been selling well - not just Finnish names but British designers, such as Gerald Summers, and Marcel Breuer."
The dealer Michael Marks, of 20th Century Marks, has also seen the market change. "When we started in this business, 20th-century design was on the edge, but now we supply a lot of the antique shops, which used to look on us as lunatics at the time, because their antiques aren't selling as well," he says.
"This sector is now much more mainstream because the furniture suits modern living better. Prices have become more stable as more dealers and auctions have arrived, but they've increased quite considerably on scarce items."
He says, for example, that while an Eames ottoman and chair could have been bought 10 years ago for £800 to £1,200, the average price is now closer to £3,000.
Specialist knowledge is important to avoid expensive mistakes. "There are things that in England were scarce such as the Garden Egg chair, from 1968, which was making £800 to £1,200," Marks says. "These are now £500 to £600 because people have realised there are more pieces around than first thought."
The best investments are the top pieces, but they can be unaffordable. Addison thinks more recent designers are worth a look. "Designers who graduated in the past 10 years or so are being included in our next sale. They include Chris Howker - his production benches are retailing at over £4,000 and we have one of the originals made for his graduation show in 1997.
"This could be a new and exciting area of the market. Furniture is becoming more like the art world in this respect. People who are interested in contemporary art are often interested in these very sculptural pieces of furniture."
Dan Tolson, of Christie's, thinks British designers and manufacturers still have a way to go. "We're finding that mid-century British furniture by designers such as Ernest Race and Robin Day is more affordable than it has been," he says.
"It's an area that reached a peak of popularity a few years ago when there was a show of Robin Day furniture, but that interest has subsided. The market hasn't developed like it has for Scandinavian and Italian."
Robin Day may be better known but people such as Robert Heritage produced good-quality work in rosewood, which was well made and undervalued. For example, a large wall unit in rosewood - which would be impossible to have made today - can be bought for less than £1,000. It should be at least £5,000 because that's what it would cost to make now.
"In the past five years, the more scarce Robin Day pieces - but not the plastic chairs such as the polypropylene chair, which is easily available - have gone up 300 to 400 per cent, and I still think there is more room to rise," says Marks.
"If I had to put money into something purely for investment, I would go for the less well known and probably more interesting British designers. It's easier for you to make a profit on something that has cost you £100 than on something that has cost £1,000."
Jeff Salmon, a dealer at Decoratum, is a big fan of South American furniture. "If you have money in the stock market, pull it out and buy Brazilian furniture from the 1950s and 1960s because it is the hottest thing, especially in America," he says.
"It's funky and, pricewise, it's at the stage that Italian furniture was maybe 10 years ago. The woods are exotic, they're so well made and perhaps the most interesting thing is that in the 1930s to 1950s there was a huge influx of Italians into Brazil and so this has come out in the designs. I'm buying from Brazil and selling back to Brazil, as well as to America. There's very little in this country."