Sure, the styling follows Audi's family overtones, the interior bears more than a passing resemblance, the controls are all in the same places and there's a generally familiar feel. Nothing strange there – except that the A6 feels like a magnified A4. It's bigger – a lot bigger, both inside and out. More legroom, more elbow room, more headroom, and the champagne-coloured interior of our A6 makes it feel a lot airier than the black cabin on the A4.
But then the A6 is a different kind of car altogether. Whereas the A4's character gave it a defined sporting edge, especially in Sport quattro form, the A6 is a serene cruiser and a more relaxed drive generally. With 155bhp, our A6 has some 25bhp less than the A4 and it shows in more leisurely performance.
That said, the 2.5 TDI's recent upgrade to 163bhp should make amends – a modification that, infuriatingly, we missed by a couple of weeks.
Our car is fitted with Audi's superb Multitronic stepless automatic transmission, which matches well the engine's torque characteristics.
Peak torque on the V6 2.5 TDI is an impressive 229lb-ft, available all the way from just 1,400rpm to 3,500rpm. Under acceleration the gearbox keeps the revs at around 2,000rpm, which gives effortless acceleration with smoothness that conventional stepped transmissions can't match.
Switching to manual shift mode gives you the choice of six ratios, with a 'Tip' action that selects up and down changes quickly and smoothly when, for example, overtaking.
In fact we've no real complaints at all on this remarkable piece of kit, except for a slight hesitance when moving off: it can catch you out on roundabouts, for example, when sometimes you need a fast getaway.
Best of all, and unlike conventional autoboxes, Multitronic does not affect the engine's CO2 output or combined consumption compared with the manual – and with emissions of just 186g/km the A6 sits in the 22% tax band under the new regime. Compare that with the A6 2.4 V6 petrol's 238g/km emissions – which place it in the 32% band – and the tax advantages of choosing diesel become starkly apparent: for a 40% taxpayer, the 2.5 TDI incurs liability of £2,451 a year, compared with a hefty £3,343 for the 2.4.
We've yet to match our car's official combined consumption of 39.8mpg, however, though the fact it has only some 800 miles on the clock probably has a bearing on this. Currently, the trip computer shows around 33.8mpg on average, though this is improving as the engine loosens up.
With our A4, consumption improved dramatically with use, ranging from just 32.5mpg on delivery to 40.9mpg at the end of our tenure – some 30,000 miles later. So I'm expecting a big improvement from the A6.
A check on CAP Monitor shows the Avant's predicted residual value over the benchmark three-year/60,000-mile operating cycle to be £9,325 or 34% of cost new. That compares with the A6's arch rival BMW 525d SE Touring auto, which shows a predicted RV of £11,300/38% on an on-the-road price of £29,985. Aside from the rational argument, I've always been a fan of the A6 Avant's understated but classy styling, and having tried several other variants I reckon the 2.5 TDI is the pick of the bunch.