1 to watch

The fourth model in the 1 Series family, the Convertible doesn't stray far from the CSI Concept which first hinted at how the whole 1 Series would look.

It has its own sharp cutting front bumper to distinguish it but there is still the BMW trademark front with the double headlamp and large kidney grilles. long bonnet, short overhangs and frameless doors.

The side view has a dominant lower swage line and a prominent shoulder line which is almost flat - a design element which BMW designers say excluded the installation of a folding hard top.

While the Convertible shares much at the front with the Coupe, the rear takes its inspiration from the nautical world, especially speedboats with a deck top leading in from the almost flat waist line.

A shaped rear boot lid tops it all off and gives more down force without the need for a spoiler.

Raising and lowering the fully automatic fabric roof (which follows closely the Coupe roof silhouette) is moderately quick (22 seconds).

The time is lengthened by having its own solid cover (unlike the Z4) which makes it more complex. But this is an operation that can be undertaken at speeds up to 30 mph.A small rear window makes rear vision a little difficult with the roof up while with the roof down the large glass area provides good visibility and wind protection although the optional wind break really is a necessity because without it there is a lot off buffeting around the neck.

The unusually low header rail does exhibit some wind noise with the top down but with it in place it transforms into a remarkably quiet and cosey cabin.

One of the options is a new anthracite fabric cover with silver flecks top give it an iridescent effect in sunlight.

BMW has managed to find a way of keeping the roof on the new 1 Series Convertible as compact as possible so it only takes away 45 litres from the boot.

With the roof up the luggage space increases from 260 to 305 litres - BMW claims enough room for two sets of golf clubs - and for added versatility there is an optional load-through hatch for longer items.

Hidden rollover protection behind the rear head rests helps to keep a clean uncluttered cabin appearance with a horizontal dashboard sweeping around the front and enveloping the front seat passengers in typical BMW style.

Equipment specification comes in three levels - ES, SE and M Sport.

The entry-level features manual air conditioning, central locking, electric windows all round, a CD radio audio system and an on-board computer.

Moving up to SE adds in automatic air conditioning, interior lighting, rear parking sensors, a three-spoke sports steering wheel, automatic headlights and Dynamic Stability Control.

Range-topping M Sport models have their own exterior styling additions and personalised interior while those specifying leather upholstery get BMW's innovative Sun Reflective technology, which keeps the hide cool.

Very quickly the Convertible family will offer five engines; a 120i and 125i lead the way this week and will be followed shortly by the rest of the line-up, which includes a single diesel.

Focus of attention is on the British-built, entry-level petrol engines, the 118i and 120i, which will eventually account for 40 per cent of the sales.

Developing 143bhp, the four-cylinder 1,995cc 118i will sell well for its 44.8mpg fuel efficiency and despite being the starter engine it is no slouch - 62mph comes up in 9.2 seconds and it will go on to a highly effective 130mph maximum.

Its partner, the 120i, uses the same basic unit but with 170bhp available, which cuts a second off the sprint time with the top speed going up to 135mph and fuel consumption going down only a fraction to 42.8mpg (emissions are 158 g/km).

Next up, and a personal favourite, is the 3-litre in-line six-cylinder 125i, which develops 218bhp for some rather spirited driving.

Sixty two miles per hour comes up in a bright 6.8 seconds with the maximum just shy of 150mph, while fuel use only drops to 34.9mpg and emissions sit below 200g/km.

This is all accompanied by a wonderful gruff rumble sound from the exhaust as the 125i performs with great enthusiasm at first and then sustains this interest in the vital mid-range delivery.

This unit benefits from BMW's efficient dynamics programme, which includes brake energy regeneration, Auto Start Stop, gear change indicator, electronic power assisted steering and active aerodynamics.

Topping the range is the twin turbo-powered 135i (also including Efficient Dynamics), which has a thumping 306bhp and produces 224 g/km with 30mpg perfectly possible - its credentials are already established as it's the current International Engine of the Year. For the record, acceleration to 62mph takes 5.6 seconds before powering its way to a maximum of 155mph (electronically limited).

Although diesel is becoming important in most sectors of the market, BMW doesn't see it making a major impact here, so the lone 177bhp 120d diesel option will only account for some 10 per cent of sales.

With the resistance to diesel convertibles gradually declining, the diesel's sprightly performance of 8.1 seconds to 62mph, 134 g/km emissions and 55mpg fuel consumption should actually make it quite appealing.

Power is transmitted to the road in all 1 Series Convertibles via a standard specification six-speed manual gearbox.

Customers can also choose a six-speed automatic gearbox.

Buyers of the 125i and the 135i variants are able to specify a more sports-orientated six-speed automatic gearbox, which has paddles located either side of the steering wheel to change gear for a more dynamic interaction with the car.

Like the rest of its siblings the Convertible is a front engine rear-wheel drive layout with a new 50/50 weight distribution to give it a typical BMW sporting attitude.

For a compliant, sporting ride and directional stability, the 1 Series Convertible has a double-joint spring strut front axle suspension with a five-link rear set-up.

Good body rigidity and a solid chassis means it has very little scuttle shake and offers drivers a dynamic platform from which it really get to grips with the performance potential - roof or no roof.

The 1 Series has the highest female driver profile of any of BMW's product range (in reality probably higher because men are often registered as the buyers but don't actually drive it) and there is an anticipation that the Convertible will be especially appealing to lady drivers.

BMW says its target model is the Volkswagen Eos although one wouldn't initially compare the two brands.

More likely is it will go head to head with the Audi A3 Convertible in a currently small sector, as it fills in a gap which has appeared in the BMW line-up as the 3 series Convertible has gradually moved more upmarket.

Until recently the market for small saloon/coupe-based convertibles has been left to the mass market players - and they have sold in big numbers in the UK. Prestige carmakers like BMW have preferred to concentrate on performance soft-tops such as the Z4 nd 3 Series.

But that is certain to change with the introduction of the 1 Series Convertible and the Audi A3 Convertible as premier brands seek new customers for their ever expanding range of cars.

Prices start at £22,335 (118i ES) rising to £32,415 for the 135i M Sport.