Until a few years ago, those considering Audi's recently facelifted A5 coupe as a company car would only have had eyes for the engine it's being tested with here. It's the 163hp diesel - the most economical option in the range. Of course, you don't need us to tell you that in recent years diesel has become deeply unfashionable, with drivers increasingly considering petrol or electrified alternatives. But would it be unduly hasty to dismiss the fuel with the A5?

In the five-door Sportback variant - two-door coupe and cabriolet versions are also available - the diesel certainly still delivers in the areas you'd expect. Compared with the equivalent 150hp petrol model, it will go 13 miles further to the gallon in official WLTP testing. It also produces less CO2 emissions, meaning it sits three BIK bands lower, saving 40% taxpayers £16 per month  in payments. However, it's not all good news, since the diesel is more expensive to buy and has weaker predicted residuals, meaning overall it's predicted to be about £0.01 per mile more expensive to run over a three-year use cycle.

Our test car pairs the diesel engine with the S Line equipment grade, which features a sporty exterior styling makeover and lowered, firmer suspension designed to improve handling. It sounds like a mismatch for the economy-focused engine option, but actually the powerplant supports engaging B-road driving rather well. It's down to its 370Nm peak torque figure, which is more than that offered by not only the 150hp, but also the 204hp petrol alternatives in the range, meaning that the car feels more sprightly than expected when accelerating in gear. It's reminiscent of the driving experience associated with modern turbo hot hatches, and perhaps signals why Audi also persists with diesel power in its S performance models. 

As with the petrol model we tested previously, the diesel S Line pays the price for that firmer suspension with a harsher ride around town, and we'd still be interested to sample the car with the regular suspension set-up. But it's more comfortable on motorways, the habitat where this car should chiefly be used, and this is an environment that suits the diesel engine too, as it's plenty strong enough for drivers' lane changing and overtaking needs, and also boasts very good refinement.

Something this engine shares with the petrol, and indeed all other engines in the regular A5 range, is a 12V mild hybrid system (the S5 performance model gets a 48V version). The 12V system's functionality is limited to allowing the stop-start system to kick in early from below around 13mph and also allowing the engine to briefly switch off when coasting downhill on motorways, a process so seamlessly managed the driver might never know it was happening if they weren't watching the rev counter. However, impressive as the system is as a standalone, it's perhaps disappointing that this is as far as electrified A5s go - there's no plug-in hybrid or even regular hybrid option.

The other highlight shared with the petrol model is the excellent interior, which features Audi's now familiar yet still very impressive 12.3in Virtual Cockpit driver display, as well as a high-quality material mix of leather, metal and glossy plastics - everything feels superbly well bolted together.

Audi A5 Sportback 35 TDI 163PS S Line S Tronic