It doesn’t happen often, I grant you, but just occasionally an idea will pop into my head a little while before someone with more money, courage, talent and vision than me actually makes it happen.

The rain-sensing windscreen wiper and the self-dimming interior mirror are good examples. I wrote about them, and they happened. Just remember that you read about auto-dip headlights here first.

My one big idea that the motor industry has failed to take up relates to the type of engine to use in large heavy off-roaders. Traditionally there have been two approaches to this, both wholly unsatisfactory.

Approach No 1 is overkill: shovel in as large a petrol engine as will fit under the bonnet and hope your target customer is too rich to notice the fuel bill.

Even if he — or, substantially less likely, she — couldn’t care less about dropping a hundred quid into the fuel tank every few miles, they’re surely going to be bothered by having to stop so often, particularly if they’re on their way to Verbier for a spot of snow.

Approach No 2 solves that problem at a stroke: fit a small diesel under the bonnet and your fuel bills will be cut, just as the distance the off-roader will cover on a tankful is extended.

The only remaining problem will be that the car is much too slow. Buy a diesel Land Rover Discovery 3 and you’ll be out-accelerated by a 1.6 litre Ford Focus. On the other hand, choose the petrol version and you’ll travel just one mile further per gallon than a Rolls-Royce Phantom.

The answer, which seems so blindingly obvious that I’m staggered it has eluded so many manufacturers for so long, is to use a large diesel such as the 3.6 litre V8 unit fitted to this Range Rover. Four years after its introduction, this generation of Range Rover finally has the engine it needed from the start.

Even before you drive it the statistics are compelling: it has the same 124mph top speed as a 4.4 litre petrol Range Rover and takes just 0.3sec longer to hit 60mph (and four seconds less than the outgoing Range Rover diesel, which had a titchy engine).

But it returns 25.1mpg instead of 18.9mpg, extending its realistic range per tankful from less than 400 to more than 550 miles. Model for model, it’s also a bit more than £1,000 cheaper and will, of course, retain its value much better. On this evidence alone you’d question the wisdom of even keeping the petrol-powered car on the sales sheets. And once you’ve driven the diesel the arguments in its favour swiftly become overwhelming. Indeed its real appeal has nothing to do with bald figures.

It lies instead in the way the car surges forward on the smallest throttle openings. There’s no need to wait for the revs to build or a lower gear to arrive, it just smoothly and silently delivers the performance you want the moment it’s asked for. For that we can thank the production of more torque at 2000rpm than a Ferrari 599 has in total.

What this engine does is turn the Range Rover into what it has always aspired but never quite managed to be: a true luxury car. Once it is warmed up and you are inside the cabin you are simply unaware that there’s a diesel engine under the bonnet rather than a petrol. There’s no rattle or harshness, just the creamy-smooth shove required to soothe your frayed nerves while you’re whisked home safely and without any apparent effort.

The least amount you’ll have to fork out for this engine is £53,120 — the cost of the cheapest Range Rover Sport fitted with it. Range Rovers (whose design, confusingly, is unrelated to that of the Range Rover Sport), with this engine start at £54,020 and rise quickly to £68,850 if you want the top-notch Vogue SE model.

Is it worth it? Talking as someone who approaches all vast off-roaders with more than a little scepticism, because of their inherently compromised dynamic abilities, I was amazed by how quickly this car earned my affection. I’ve been driving this generation of Range Rover since its introduction, and unlike most of my colleagues in the motoring media I never felt more than lukewarm towards it.

If you do a jigsaw and find at the end that a piece is missing, it doesn’t matter what the picture is because all you can see is the remaining space. Four years late, the Range Rover jigsaw is complete, and only now has it revealed what an attractive and compelling picture it is.

THE OPPOSITION

Model VW Touareg 5.0 SE TDI £53,285
For Powerful engine
Against Expensive, not much fun to drive

Model Mercedes ML 320 CDI £37,220
For Good blend of power and economy
Against Proper off-road ability costs extra