The first deliveries of the latest version of the UK’s most popular Audi began on March 16.

The A3 is Audi UK’s largest volume model line, accounting for around one in every four of the brand’s new car sales here, and the A3 Sportback, the version with five doors, is the key model, capturing three-quarters of the demand.

Martin Sander, Audi UK director, said after the brand’s 10,000 unit growth in 2012 and continued rise through January 2013, the arrival of A3 Sportback will further underpin the momentum.

The new A3 Sportback costs a £620 premium over the three-door A3 and, alongside the easier access for rear passengers, buyers will benefit from a slightly more spacious interior thanks to a wheelbase 35mm longer and 30 litres more bootspace.

It also gets rear electric windows and rear door child locks, due to being more practical as a premium small family car.

Otherwise, its equipment levels mirror the three-door A3 across the three SE, Sport and S-Line trims, with the basic spec including a retracting colour screen, Bluetooth, 16in alloys, electronic handbrake, air-con and cloth seats.

Initially A3 Sportback is on the market with three engine options: a 148bhp 2.0-litre TDI diesel and two TFSI petrols comprising of a 120bhp 1.4-litre and a 176bhp 1.8-litre.

The diesel is expected to take the majority of demand, due particularly to the fleet sector’s interest in A3 Sportback – 60% of units end up in the hands of company car drivers.

However, for private buyers unlikely to cover 14,000 miles per year the 1.4TFSI Sport at £21,050 makes total sense.

It’s a superb, lively engine with reasonable efficiency and fine equipment, and the £2,300 buyers save by snubbing the equivalent 2.0TDI could pay for parking sensors, leather seats, sat-nav and metallic paint – livening up the otherwise staid interior.

The range will be expanded in May with the addition of a 104bhp 1.6TDI priced from £20,775 with official figures of 99g/km CO2 and 74.3mpg. It’s the most efficient engine in the range.

Other additions this summer will include a 139bhp version of the 1.4TFSI with a cylinder on demand system which deactivates two cylinders at low loads and while coasting, to improve fuel efficiency to 60mpg.

A 1.2TFSI with 104bhp and combined fuel economy of 57.6mpg, and a 180bhp 2.0TDI, capable of 64.2mpg and 0-62mph in 7.4 seconds, will be added later in the summer.

Like all premium brand products, the A3 Sportback has an extensive list of optional equipment.

Some have been bundled to ease the sale and add value, such as the Comfort Package which is expected to be most popular and includes auto wipers and lights, rear parking sensors, cruise control and an auto-dim mirror, or the Technology Pack with a hard-drive based navigation, music and DVD system, voice control and touch-sensitive multimedia interface (MMI).

Others are individual add-ons, from DAB radio and illuminated vanity mirrors to pre-sense basic which closes the windows automatically if it senses a crash is imminent.

To encourage customers to stay within the franchised network, Audi also promotes a £399 service plan covering three years or 30,000 miles and warranty extensions up to five years or 90,000 miles.