Hot plate and Spork fight it out for best gadget prize

Self-heating crockery, a clock that tells the time in colours, a light-sensitive window blind – welcome to the world of things you never knew you needed but one day might be queuing up to buy.

These and more than 100 other ingenious devices are some of the fruits of a new boom in "garden-shed" inventing which will be shown at the British Invention Show, beginning on Thursday at the Barbican Centre in London. Each will be competing for the prestigious Invention of the Year award: the greatest accolade for Britain's small army of amateur or "lone" inventors.

This year's contenders include Mark Champkins, 25, an engineering graduate from Cambridge University, who has designed a range of self-heating crockery. Mr Champkins, who got his idea from the chemicals used in commercial hand-warmers, firmly believes his invention has a realistic and practical future. "I'd like my idea to be used properly, in places like hospitals, canteens and schools – anywhere there is hot food in a cold environment," he said.

Mr Champkins claims that his device, which can heat bowls, plates or mugs up to temperatures of 60C, can keep food and drink hot for three times longer than normal. "Each year around £45m is spent on hospital food that ends up being thrown away," he said. "This invention can ensure that the food is still warm when the trolley reaches the end of the ward, and all the patients have finished eating." The secret of Mr Champkins's invention is a simple chemical reaction, which crystallises the sodium acetate trihydrate stored at the base of the crockery. "You can put your bowl or plate into the dishwasher, or just into hot water, and it's recharged and ready to go again," the inventor said.

There will be stiff opposition from a number of other innovations at the show, including a self-dealing pack of cards; the "Zipkey", a combined zip and padlock for luggage; a bidet fitting for standard toilets, and the "Spork", a spoon-fork hybrid.

Another contender is David Wilks, 50, a technology teacher from Mirfield in West Yorkshire, with his "Varyflush" toilet. Standard toilets use nine litres of water for each flush, whereas Mr Wilks has calculated that four litres is normally sufficient. His invention, based on an interruptable siphon system, gauges the amount of water to use on each flush. It is a system which he believes could save most British households 50 per cent of their annual water bill – a total consumer saving of £750m. He claims that if everyone in the UK had a Varyflush system, it would save 456 million tons of water a year, as well as 262,000 tons of carbon dioxide emissions and 550,000 megawatt hours of electricity.

Last year's Invention of the Year award, which is judged by the Institute of Patentees and Inventors (IPI), was won by Mark Sheahan, a former IT consultant from Bromley, south London, for a plastic "squeeze to open" container.

Some that worked ...

Pocket calculator: In 1971, Clive Sinclair visited Texas Instruments in America and returned to Cambridge with a revolutionary circuit chip allowing computerised calculation in compact form. Shortly afterwards, he had invented the world's first pocket calculator.

Viagra: In 1991, scientists at the Pfizer laboratories in Kent stumbled on a discovery that brought hope to millions of men of a certain age. During research into a drug to treat angina, they found that their pills had an effect on the biochemical processes involved in male arousal. Viagra was born, and, with it, a new generation of awful headlines.

Hovercraft: During the 1950s, the radio engineer Christopher Cockerell began experimenting with the idea of using a cushion of air to reduce a craft's friction across water. By 1960, he was demonstrating his first hovercraft to government experts and soon afterwards a prototype crossed the Channel. Sir Christopher went on to become one of Britain's most prolific inventors, with 90 patents registered by the time he died in 1999.

And some that didn't ...

Winged golf ball: In 1968, the Sussex inventor Arthur Pendrick had the idea of a golf ball with flaps. Attached to the surface of the ball by magnets, the small wings were supposed to use centrifugal force to control the spin. The invention itself never took off.

Gender-sensitive car number plates: A 2000 patent awarded to Duncan Butlin proposed colour-coded number plates differentiating drivers according to gender. Butlin's premise was that "people can predict the behaviour of others if they know their sex". He suggested that men have white number plates and women yellow, "because yellow is a prettier colour".

Spider ladders: Edward Doughney patented the Spider Ladder in 1994. Designed as an escape route for arachnids stuck in baths, the ladder had a thin latex rubber strip that could be attached near the taps. It never achieved its commercial potential.