There was a moment, right after passing the 140mph mark on Bruntingthorpe's back straight, when I began to realise just how far the Sin R1 road car has come in its short life.

I last saw this V8-engined, 444bhp sports car at Autosport International in January and, to be honest, was a little disappointed not to see a finished product. With the R1 racing car already finished and ready to race in the GT Cup, I'd expected to see the full R1 road car – but instead the firm showed a hollow prototype, a body with no engine or interior.

Nevertheless, nine months later an almost-complete car is hurtling around the Leicestershire aerodrome. I say ‘almost-complete’, because although orders are being taken this is still described as a pre-production model. It's about 95 per cent there, Sin boss Rosen Daskalov tells me. The interior, mechanicals, shape and panels are as customers would see them. What remains is to lift the fit and finish.

There's some work to do in that area, because although the R1 looks very striking on the road, close inspection reveals some panel gaps which are too large, an uneven paint finish in places and carbonfibre trim which looks pasted on.

It'll all need attention if Sin wants to succeed in attracting the kind of clientele who have £72,000 to spend on a sports car. That’s high-end Jaguar F-type and Porsche 911 Carrera money, after all.

The real proof of the car's potential, though, is in a passenger lap. I'm told the R1's Michelin tyres have already been put to good use on a cross-continental dash from Bulgaria, where the Sin company is based. Not helping is the British weather, and Bruntingthorpe is seriously wet by the time we head out.

Getting over the R1's high door sill takes finesse, but once strapped into the relatively spartan cabin the visibility is more than adequate. Though narrow, those leather sports seats are comfortable, too. Driver Jonny MacGregor straps into the other side, and as we head towards the circuit he explains this is the first time he's ever driven the R1 road car.

The 6.2-litre V8 engine's sound fills the cabin – more so than in other similar cars, it has to be said. There may be some sound deadening work to do here.

Soon enough we're on track, and it's clear what chief executive Daskalov and his team have spent their time doing. The R1 feels smooth and comfortable, even as we enter triple-digit speeds.

MacGregor works the six-speed manual transmission hard, and it seems a little notchy at first, but the car's response and handling appear to be pitch perfect. Later Daskalov tells me he rarely uses first gear as it’s so short, and instead pulls off in second. 

On the back straight the R1 hits 140mph with no trouble. It almost feels lazy, effortlessly cruising along, but I'm reminded that earlier in the year we hit the same speed here in an old £500 Jaguar XJ Sport. There’s some dancing about from the front end and lots of wind noise, but from the passenger seat it still feels impressive.

A specific type of customer will be interested in buying an R1. With the racing version helping to raise brand awareness, there'll be those who can not only afford to buy the model, but who like the idea of owning truly rare sports car.

Plus, it has to be said, getting a project from hollow shell to full sports car in nine months is an achievement, especially for a small group working out of both the UK and Bulgaria. 

The first cars could be ready for customer deliveries at the end of the year, leaving the team ample time to work out the final kinks. If they succeed, R1 owners could end up with a car capable of fulfilling its promise of offering supercar thrills at sports car prices.

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