But with an IQ of 159, millions of pounds in the bank, lifetime Mensa membership and a cult-like status as one of the most famous British inventors of the 20th century, Sinclair does not, it must be said, seem your archetypal idiot. Then again, he did create the C5 – a plastic, snail-paced vehicle, a bit like a pram and Robin Reliant hybrid – and nearly ruined his business, reputation and entire future in the process.
Sinclair came from an inventive family. His grandfather, George Sinclair, was a naval architect involved in the creation of the paravane, a minesweeping device, while his father, Bill, was a mechanical engineer with his own machine tools business. Sinclair was born in 1940, not long after the outbreak of the Second World War, near Richmond in Surrey. He and his mother were evacuated to Devon, and their home in Richmond was later bombed.
When Sinclair was in his teens, his father's business ran into financial difficulties resulting in Sinclair having to move school several times. For the shy, geeky boy who was a whizz at maths (he took A-levels in physics, pure maths, and applied maths) and felt awkward with other children his own age, it was a difficult time. Sinclair immersed himself into the world of adults, spending time with his family and taking jobs at electronics companies during the holidays.
Instead of going to university – an obvious choice for someone with such obscure and focused A-levels – Sinclair went into business, selling miniature electronic kits by mail-order. He even found time to write books with snappy titles such as Practical Transistor Receivers Book One, and in 1961 he set up Sinclair Radionics after designing a miniature transistor pocket radio. The following year he married Ann Trevor-Briscoe, and they later had three children, Belinda, Crispin and Bartholomew.
Sinclair has a well-documented fondness for small things – small pocket calculators, small watches, tiny portable TVs – that is perhaps connected to the fact that he is not, himself, a tall man. Whatever the reason, he soon got to work on the pocket calculator, which he launched in 1972, and the world's first portable television, which he launched in 1977. He ran his company out of Cambridge – the area is the Silicon Valley of Britain thanks in large part to his connection to the town – and sold a range of products, most in electronic kit form.
But although he was fairly successful, it wasn't until 1980, when he unveiled the Sinclair ZX80 computer, named after the year it was created and because the letters "sounded cool and futuristic", that Sinclair truly hit the big time. The ZX80 cost 79.95 in kit form or 99.95 fully built – a bargain compared with other computers on the market, particularly given that it rather ambitiously promised to do "quite literally anything, from playing chess to running a power station".
It was a huge seller, and following the launch of its successor, the ZX81, briefly the biggest selling computer in the world. For Sinclair, who describes himself as "the eternal optimist", it was an exciting time but he tried not to get carried away.
"You can meet with triumph and disaster," he remarked. "I don't get too high when it's supposed to be looking good and I'm not knocked down when it's not looking so good."
In 1983 he was knighted on the recommendation of then Prime Minister Margaret Thatcher – who viewed him as a pioneer of new light industry and regularly invited him to tea at Number Ten. Some saw him as a saviour, a clever nerd who would lead Britain into a new technological age, a Bill Gates of his time.
But in 1985, it all went wrong. He launched the Sinclair C5, a vehicle that promised to save energy and get the nation to work on time. The nation took one look at it and burst out laughing. The vehicle flopped. The fiasco cost Sinclair an estimated 7 million. Meanwhile, his marriage to Ann was in trouble and later that year the couple divorced.
For a while all went quiet in the Sinclair camp, as he retreated to lick his wounds. He admitted recently that the C5 saga had been dealt with badly. "Clearly I should have handled it differently. If I had it could have succeeded. I rushed at it too much and invested too much in the tooling and I should have gone a bit more gently into it."
In the 1990s, he invented a bicycle battery for wheelchair users, and continued to make himself surprisingly available to an always-curious public. As late as 1996, his telephone number was still listed in Directory Enquiries and he was forever being called by wannabe inventors looking for advice. He also developed a reputation as a ladies' man, and was seen with a number of women, most of them much younger than him. "He is very attractive to women," one winsome young lady told a disbelieving public.
In 1996 he met Angie Bowness, a Stringfellows table dancer and, at 21, 37 years his junior. He was smitten. "Falling in love is irrational," he said at the time, employing the sort of boffinry for which he was now rather famous. "It is not a question of reason. The subconscious brain decides you are going to fall in love and pumps chemicals into the conscious brain and there you are."
But Bowness left him for someone else (she liked All Saints and going out, Sinclair liked classical music and staying in) and for years, Sinclair was alone again. They reconciled in 2008, and got engaged last year. Bowness says she always thought they would get back together, and he describes her as the love of his life. Last month, they tied the knot in Las Vegas.
The pair divide their time between a Lincolnshire farmhouse and Sinclair's opulent apartment, which overlooks Trafalgar Square. Sinclair, is loving stepfather to Bowness's son, Marcus, while his own children are all older than his wife.
And he's still inventing. In 2006 he brought out the A-bike, a lightweight folding bicycle designed for commuters, and in a recent interview he said he had now turned his attentions to "a little electric car" which he hoped to make available within the year.
"As usual I hope I'll sell lots of them," he said of the project, which sounds suspiciously like a souped-up C5. "But who can tell?"
Wise words from an old, but clearly happy, fool.