Normal rules do not apply, according to Vauxhall's forthcoming advertising campaign, and one look at the new Signum confirms its sector-busting credentials.

The Signum is part of Vauxhall's attempt to create ground-breaking niche vehicles offering unrivalled flexibility and appealing to those looking for clever solutions to the problem of combining passenger comfort with luggage capacity.

Full details of Signum prices and specifications were revealed at the Geneva Motor Show three months ago, but last week we had the first opportunity to drive it.

Based on the Vectra, the Signum is a 'premium' car, which Vauxhall hopes will steal sales from vehicles such as the Alfa Romeo 156, Saab 9-3, Volvo S60, Lexus IS and top-end Volkswagen Passats.

Apart from a modified front end, the Signum is basically Vectra as far as the B-pillar. However, like the forthcoming Vectra estate, the wheelbase is stretched by five inches. And the rear compartment of the Signum is the part that offers something genuinely different.

The rear seats in the Signum have a 40/20/40 split, with the outer sections able to slide forwards and backwards, and also recline to a degree.

Elite models have a 'travel assistant' as standard, which acts as a large centre armrest in the rear, incorporating storage compartments which can be specified with a portable DVD holder and cool box.

Signums can also be equipped with adaptive forward lighting which works in conjunction with the steering to light around bends, while reflectors offer better left and right visibility at night at junctions.

The Signum also debuts a new range of engines for Vauxhall. First in the range is a new 2.2-litre direct injection petrol engine offering 153bhp. A 2.0-litre turbo used in the Saab 9-3, developing 172bhp and with a broader torque band is also available.

The other new engine is a 3.0-litre V6 CDTi, Vauxhall's first common rail diesel, developing 174bhp and 273lb-ft of torque. These engines are joined by a 3.2-litre V6 petrol engine and two diesels – a 2.0 DTi and 2.2 DTi – all of which have been available with the Vectra since it was launched nearly a year ago.

Although none were available to drive on the launch event, I question the wisdom of offering a 99bhp 2.0 DTi. It might offer low emissions, but I imagine it will feel feeble in such a large car.

Vauxhall expects to sell 8,000 Signums in a full year, and 75% of sales will be to fleet customers. However, the term user-chooser and Vauxhall are rarely used together when list prices reach more than £25,000.

Vauxhall brand manager fleet Paul Adler said: 'We think there are enough drivers who will not be swayed by badge snobbery. There are people who would rather not drive a premium badge German car and 8,000 units a year when cars like the BMW 3-series sell 50,000 puts it into perspective.'

Vauxhall's marketing people claim enough people will be won over by the practical and versatile features of the Signum, and for the models sales targets I have no reason to doubt their research.

CAP Monitor's residual value data shows low-end models should retain 35% to 36% of their value after three-years/60,000-miles, with the top-end V6 models retaining about 29% to 30%, which leaves it in touch with the semi-premium cars it aspires to compete with.

Far harder to swallow is the price premium paid for the 3.0 V6 diesel. Buyers are being asked to part with nearly £3,000 more than for the petrol 3.2-litre V6, which is ludicrous.

Either the engine is made of gold, or Vauxhall has over-estimated the appeal of the Isuzu unit, also used by Saab and Renault. Generally manufacturers cheekily ask for a premium of about £1,000 to £1,500 for a diesel model over a petrol equivalent in the UK.

But for Vauxhall to demand a premium of £2,750 is over-optimistic, particularly when the same diesel engine offered by Saab is £300 cheaper than the petrol 9-5 3.0 V6.

How long before Vauxhall revisits this issue and drops prices to a sensible level? In spite of this aberration, the Signum is a refreshing change for the sector – that shady area between upper- medium and premium upper- medium that is the site of an increasing number of scraps for volume manufacturers. It is endowed with a level of all-round competence and offers the genuinely innovative packaging that is rapidly becoming a Vauxhall trademark.

Behind the wheel

THE Signum stands out from the crowd in a way the plain- looking Vectra never could.

At the front it incorporates the deep front spoiler of the Vectra GSi along with fog lamps placed on chrome-effect bars.

But its profile is far more distinctive with its long roof line and extended wheelbase. The wrap-around tailgate glass merges seamlessly with the rear light clusters and the hatch cuts deep into the rear bumper for easy loading and unloading of heavy objects.

My first experience of travelling in the Signum was as a rear-seat passenger from the outskirts of Manchester to the city centre. Stretched out in the comfortable armchairs in the vast rear compartment was a relaxing experience, while viewing a promotional video on the portable DVD player plugged into the travel assistant.

The sliding rear seats have two benefits. When they are not being used, they can be pushed forward to increase the boot volume and the seatback can be reclined by up to 30 degrees to allow passengers to unwind in comfort.

I did feel that leather-trimmed Elite models were more appealing than the entry-level cloth-trimmed Elegance cars, which were too much like a standard Vectra and didn't quite feel special enough.

Driving the Signum is not dissimilar to the Vectra. The steering is not quite as positive as I would like, but it is relatively easy to hold a chosen line through a series of bends without adjusting the wheel.

The most impressive feature is the ride quality and refinement. It is very difficult to unsettle the car, even on the worst road surfaces and each of the new engines we tried behaved impeccably.

Acceleration in the 2.2 Direct is sprightly – it remains close to the 2.0T in the 0-60mph sprint – but it is muted except when revved hard. Maximum torque of 162lb-ft is more than adequate and it begins to pull from around the 2,000rpm mark.

You notice the difference in performance of the 2.0T at low revs as it accelerates with far greater urgency. With a six-speed manual transmission – largely unnecessary – the 2.0T has a surprising turn of speed with maximum torque available from 2,500-3,800rpm. The latter figure is the point where the 2.2 Direct is just getting warmed up.

Although we are familiar with the 3.0 V6 CDTi from various Saabs and Renaults, it is at its most impressive in the Signum. With 273lb-ft of torque (as opposed to 258lb-ft in the Saab 9-5, Renault Vel Satis and Espace) it provides fuss-free rapid acceleration with hardly any noise intruding into the cabin.

During a sprint along a slip-road to join the A55 outside Llandudno the Signum sounded just like a petrol V6. Factor in 38mpg combined fuel economy and this could be an excellent Grand Tourer. Shame about the price though.

Driving verdict

The Signum looks distinctive from most angles and feels special when you sit in the rear. It is not particularly engaging to drive, but is supremely refined, spacious, comfortable and versatile.