Size does matter, at least in the case of the Audi A3. According to its maker, the addition of just 3.3in in length “transforms” the regular A3 hatchback into something grandly named Sportback.

Introduced to the UK in September 2004, the Sportback is so named because it has five doors, and as a consequence occupies a unique position, bridging the gap between people who need small-estate practicality and those who prefer not to admit that they do.

The wheelbase is identical to that of the three-door A3 and the VW Golf, but the Sportback’s repositioned rear seat liberates extra rear legroom and the addition of back doors eases rear-seat access.

The extra overall length also produces increased boot volume - up to a healthy 370 litres - although it is worth noting that all-wheel-drive quattro models have a boot limited to 302 litres because of the rear drivetrain.

Engines pretty much mirror those of the three-door A3, with a good mix of diesel and petrol. The entry level 1.6 petrol comes in 102bhp or 115bhp guise; the latter being fuel-efficient direct injection (FSI). Neither provides much performance but you still get that Audi feelgood factor. For about £1,000 more, though, buyers can enjoy a used 2 litre FSI with 150bhp. It’s smoother and nearly 2sec quicker to 60mph, the marginal 4mpg dip in fuel economy being easy enough to justify.

There is also a turbocharged 2 litre FSI producing 197bhp – the same unit as found in the Golf GTI – and unsurprisingly, it makes for an impressive drive and helps to justify the Sportback moniker.

Top of the range is a 236bhp 3.2 litre V6 that’s mated to quattro all-wheel drive. With a 0-60mph time of around 6.7sec it’s certainly quick, although with a new list price of about £25,000 (before options) it’s not a car that has sold in significant numbers.

It’s the diesel A3 models that are the volume sellers: the ubiquitous 1.9 TDI provides entry level power and performance with its modest 105bhp mated to sparkling 50mpg-plus economy.

Next in line is the 140bhp 2 litre, which has particularly strong midrange performance and better refinement than the 1.9, and was the biggest seller until May 2006, when Audi introduced an uprated 170bhp version. While used prices have yet to dip much below £15,000 for models with this engine, it seems they are becoming the “sweet spot” of the range.

Having decided on your engine, further consideration must be given to specification and trim. While all used buyers can be assured of enjoying Audi’s superb build, entry level cars can feel a bit spartan and you need to go for SE or Sport specification to get digital climate control and alloy wheels.

With the option lists bulging with tempting extras, many secondhand cars may boast nonstandard kit such as a stereo upgrade, satellite navigation and leather upholstery. But let the buyer beware, for expensive factory options add little to secondhand values: so you should pay only about £300 extra for sat nav, £400 for leather trim, and nothing for upgraded audio.

On the road the A3 Sportback is typically Audi: refined and surefooted, with a beautifully executed interior. The driving experience is heavily dependent on the chosen power plant, but all models fare well when compared with similarly priced machinery.

The allure of quattro all-wheel drive can be strong, but in normal driving it is unlikely to make itself felt, and buyers pay the price for it in terms of the increased fuel consumption and reduced boot space.

The Sportback name was dreamt up to appeal to a marketplace obsessed with niche lifestyle genres. Four years on and you have to wonder whether Audi considers the name to have lived up to the car’s abilities; I guess we will have to wait and see – when the upgraded A3 arrives later this year.

Alloy wheels Standard on all but base model
Interior Cloth seats are durable, sports seats are superbly supportive
Engines Petrol 1.8 litre FSI turbo engine with 160bhp has now replaced 2 litre FSI
Satellite navigation Don’t pay more than £300 extra for it
Safety Four-star Euro NCAP rating for adult occupancy; three stars for child occupant safety
Boot 370 litres on two-wheel-drive models but only 302 litres on quattro
Insurance Group 9-15 on most models but 18 on the high-performance 3.2 litre V6
Gearbox Manual gearbox works well, but superb DSG gearbox (renamed S tronic), is the pick of the bunch for its super-rapid automatic changes and the additional ability to shift manually
Roof rails Optional matt or aluminium roof rails give the Sportback small-estate looks
Security Alarm, deadlocks and immobiliser are standard on all models
Stereo Standard CD unit is of good quality; upgraded Bose units with better performance were an option, but don’t be tempted to pay a hefty premium for one

Vital statistics

Model A3 Sportback 2.0 TDI S-line
Engine 1968cc, four cylinders
Power 140bhp
Transmission Six-speed semi-auto DSG
Fuel consumption 48.7mpg (combined)
Acceleration 0-60mph: 9.2sec
Top speed 129mph
CO2 emissions 154g/km
Road tax band D (£145 for 12 months)
Cost new today £24,055 (S tronic gearbox)

The one to buy
A 2005 55 Audi A3 Sportback 2.0 TDI S-line DSG with 20,000 miles.
Pay £18,250 at a dealer or £17,000 privately

Values

Mileage (10,000/20,000/40,000)

2004 54
Trade £15,550 £14,850 £13,350
Retail £17,950 £17,195 £15,695

2005 55
Trade £16,800 £16,000 £14,400
Retail £19,150 £18,250 £16,750

2006 56
Trade £18,100 £17,200 £15,550
Retail £20,500 £19,550 £17,895

Or for similar money
2006 55 M-B C 220 CDI SE Sports Coupé auto
2007 56 BMW 120d Sport Steptronic
2007 56 Volvo S60 2.4D SE Geartronic
2007 07 Saab 9-5 2.3 Hot Aero estate auto
2007 07 Volkswagen Golf 2.0 GT TDI