After a change in the law, prompted by lobbying from bicycle manufacturers, Sinclair developed the C5 as an electrically powered tricycle with a polypropylene body and a chassis designed by Lotus Cars.

It was intended to be the first in a series of increasingly ambitious electric vehicles, but the development of the follow-up C10 and C15 models never progressed further than the drawing board, mostly due to the poor public response to the C5.

The vehicle's limitations – a short range, a maximum speed of only 15 miles per hour (24 km/h), a battery that ran down quickly and a lack of weatherproofing – made it impractical for most people's needs.

Enthusiasts have established owners' clubs and some have modified their vehicles substantially, adding bigger wheels, jet engines, and high-powered electric motors to propel their C5s at speeds of up to 150 miles per hour (240 km/h).

[10] These included indicator lights, mirrors, mud flaps, a horn, and a "High-Vis Mast" consisting of a reflective strip on a pole, designed to make the C5 more visible in traffic.

Sinclair's C5 accessories brochure noted that "the British climate isn't always ideal for wind-in-the-hair driving" and offered a range of waterproofs to keep C5 drivers dry in the vehicle's open cockpit.

Around Christmas that year, he approached Tony Wood Rogers, an ex-Radionics employee, to carry out consultancy work on "a preliminary investigation into a personal electric vehicle".

The maximum legal speed of the vehicle would be limited to only 15 miles per hour (24 km/h); it could not weigh any more than 60 kilograms (130 lb), including the battery; and its motor could not be rated at any more than 250 watts.

[26] A prototype was presented to 63 families in the A, B, C1 and C2 demographic groups in suburban and town environments to determine that the controls were correctly positioned; this was the only external research carried out on the C5.

[25] The development of the C5 took place over 19 months in conditions of great secrecy, with testing carried out at the Motor Industry Research Association's proving ground in Leicestershire.

The event was staged in Sinclair's usual glitzy style, with women handing out press packs[35] and a variety of promotional giveaways: magazines, hats, pullovers, T-shirts, key rings, sun visors, badges, mugs, bags, and even a C5 video game.

[36] Sinclair issued a glossy sales brochure characterising the vehicle as a part of an ongoing exercise in "cutting giants down to size, turning impersonal tyrants into personal servants".

"[5] The photographs accompanying the text showed housewives and teenagers driving the C5 to shops, railway stations, and sports fields – in the words of technology writers Ian Adamson and Richard Kennedy, "a blue-sky suburbia exclusively populated by electric trikes and their drivers".

[42] The Sunday Times called the C5 a "Formula One bath-chair"; its reporter "had travelled five yards outdoors when everything went phut and this motorised, plastic, lozenge rolled to a halt with all the stationary decisiveness of a mule".

"[7] The timing and location of the launch event – in the middle of winter, on the top of a snow- and ice-covered hill – later prompted criticism even from Sinclair executives, who admitted off the record that spring conditions might have been better for a vehicle with so little protection from the British climate.

[47] Sinclair's biographer Rodney Dale describes it as "a calculated (or miscalculated) risk", observing that production was already underway, details were beginning to leak out to the press and "the launch could hardly have been held up until the possibility of a bright spring day".

"[45] The Guardian's motoring correspondent wrote of her "grave misgivings about its use in congested traffic ... On a sharp turn it too easily lifts a rear wheel, is hazardously silent, and low down.

They included holiday camps who wanted C5s to rent to campers;[58] the British Royal Family – Princes William and Harry each had one to drive around Kensington Palace before they were old enough to drive;[60] Sir Elton John, who had two;[61] the magician Paul Daniels, who bought a demonstration model he saw being driven around the BBC Television Centre car park;[62] Sir Arthur C. Clarke, who had two shipped out to his home at Colombo in Sri Lanka;[63] and the Mayor of Scarborough, Michael Pitts, who swapped his official Daimler for a C5.

[67] Adding to Sinclair's problems, production of the C5 had to be halted for three weeks after numerous customers reported that the plastic moulding attached to the gearbox was impairing the performance of their vehicles.

[69] Possible sales opportunities were explored in mainland Europe, Asia, and the United States, with Sinclair claiming that he had found "very big" levels of interest.

[73] The magazine also called the C5 "too easy to steal",[74] hardly surprising considering that while a security lock could be used to prevent it being driven away, the C5 was light enough that a would-be thief could simply pick it up and carry it off.

[81] The last of the unsold C5s were bought for £75 each by Ellar (Surplus Goods) Ltd of Liverpool, which planned to sell 1,000 of them to an Egyptian businessman for use on a university campus while another 1,500 were intended to be sold in the UK.

"[47] Ellar's director Maurice Levensohn took exactly this tack when he purchased Sinclair Vehicles' remaining stock, saying that his company would market them as "a sophisticated toy": "If you were a little boy, wouldn't you want your parents to get you one this Christmas?

[3] Andrew P. Marks of Paisley College of Technology criticised Sinclair's marketing strategy as confused; the C5 promotional brochure depicts it as a leisure vehicle, showing boys in C5s at a football pitch, women in C5s on a suburban road, and so on, while the text suggests that the C5 is a serious substitute for a car.

[30] He comments that the experience of working on the C5 convinced him of the need for industrial designers such as himself to get "involved early in the innovation process, shaping basic configurations, never again [being] satisfied to simply decorate a fundamentally bad idea".

"[61] Sam Dawson of Classic and Sport Car Magazine described the C5 as "incredibly fun to drive", suggesting that the safety concerns "could have been addressed if it wasn't for the fact that it was already doomed as a national joke."

[83] Professor Stuart Cole of the University of South Wales comments that the C5 suffered from the design of the roads and the attitudes of the time, which were not geared towards pedal or electric vehicles: "In the days before unleaded petrol, your face would have been at the height of every exhaust pipe, and drivers weren't used to having to consider slower-moving cyclists.

But with more cycle lanes, better education, and workplaces providing showers, etc., the world now is much more geared up for people looking for alternatives to the car, and hopefully will become even more so in the future.

The design is strikingly similar to the modern Renault Twizy electric vehicle; Wood Rogers comments that "you could put the C10 into production today and it would still look contemporary.

"[27] At the time of the C5's launch, Sinclair described the C15 as having "a futuristic design with an elongated 'tear-drop' shape, a lightweight body made of self-coloured polypropylene and a single, possibly 'roller' type rear wheel".