Discuss: is a hybrid engine best used to make a large inefficient engine more efficient, or should it be used to make a smaller, already efficient engine very efficient?

Within the Toyota Lexus group, the approach has been two-fold: Toyota is using hybrids in its Prius and Auris cars to make models that would already be pretty efficient extremely frugal.

Lexus, meanwhile, is putting hybrids into its massive SUVs and executive carriers that, as petrols, would be gas guzzlers but as hybrids are relatively economical.

Note the words ‘as petrols’. Stick a powerful diesel into an LS600h rival and you can expect efficiency in the mid-30s and CO2 around 200g/km. The LS600h? 30.4mpg and 218g/km.

The other issue with the LS600h is its running costs. The car has always been excluded from the Fleet News Awards at an early stage because its running costs are so much higher than its rivals.

OK, so this is partly down to the high P11D price explained by Lexus’s preoccupation with offering exceptional levels of specification on the £89,955 LWB model (no options available).

But can any fleet justify 113.87ppm over four years/80,000 miles when the (lower equipped) A8 4.2 LWB diesel (204g/km, 36.2mpg) offers 86.81ppm? And if image rather than out-and-out grunt is your preference, what about the BMW 730d LWB (180g/km, 40.9mpg) at 76.02ppm over four years/80,000 miles?

To be fair, when the LS hybrid was conceived, there was probably a stronger presence among businesses and chauffeur fleets of 12-cylinder petrol luxury saloons (the versions that the V8 + electric motor Lexus is targeting), than there is now in these more austere times, where six-cylinder diesels have increased their stranglehold on the luxury sector.

Set aside concerns over running costs and the perception of the LS600h as hybrid’s answer to the eco-friendly luxo-barge, and the car is very capable.

It’s a Lexus so you can be sure of maintenance-free motoring – something not always taken into consideration by the SMR figures. Neither are the lower tyre and brake costs thanks to the hybrid technology and eco-tweaks. Each should contribute to a lower running cost total than the official figures suggest.

The LS600h is roomy with class-leading levels of comfort. The interior focus is on a wood and leather combination for the trim while the controls still hark from an era where knobs and switches rule over the iDrive and MMI interfaces of the BMWs and Audis, although the touchscreen sat-nav helps to bring the car into the modern age.

The engine is generally refined and the car is surprisingly agile when required, although the ability to toast the twisty bits will not be the clincher on buyers’ lists.

We didn’t have the relaxation pack on our test car – a £9,500 extra (treated by Lexus as a separate model) – which, if specified, gives rear seat occupants Maybach levels of comfort and space on one side of the rear compartment at a fraction of the price.