Peer down at the console between our long-term Phase 2 Renault Kangoo Sport’s front seats and you will see a little button marked ECO in slightly-faded type. Press it, and a green light comes on – and your fuel bills hopefully start going down.

That’s because using the button has an immediate impact on pretty much everything on the vehicle that consumes power. It limits the available torque, changes the accelerator pedal’s mapping and alters the gear shift indicator so that you are encouraged to change up to a higher gear earlier than you might otherwise do.

It has an impact on the heating and air-conditioning systems too so that they burn less energy. That means the temperature in the cab can be kept comfortable; neither boiling hot nor (and this is the option the writer typically chooses) so bitterly cold that ice forms on the inside of the windscreen and polar bears rap firmly on the van’s back doors demanding immediate entry.

So does the ECO button work? Does it deliver the up-to-10%-saving in diesel usage that Renault claims for it?

The answer is yes if you make a conscious effort to adapt your driving style just as soon as you have pushed it; which in effect means behaving in the way you should be behaving at the wheel no matter whether you have pressed the button or not.

That means driving smoothly, not speeding, not switching off the Stop & Start system, changing up a gear whenever it makes sense to do so and not accelerating harshly.

If you stamp hard on the accelerator pedal then the ECO system automatically switches itself off, but cuts in again the minute you lift your foot off. If that sounds something of a paradox then it must be admitted that even the most frugally-minded van drivers occasionally have to accelerate themselves out of trouble and thus require an ECO over-ride.

Rather more of a paradox is the impossibility of activating cruise control (an extra-cost option on our demonstrator) if  ECO mode is operational.

Using the ECO button has minimal effect on performance if you are driving through flat terrain either empty or lightly-laden. The 1.5-litre 90hp diesel burbles away quite happily to itself and seems at one with the world.

Heave so many filled sandbags into the 3.0m3 cargo area that you are making full use of the van’s 600kg gross payload capacity, then tackle some of the steeper hills in the Forest of Dean in Gloucestershire, and it is a different story.

The engine starts to breathe hard, your pace slows and you are conscious of a growing queue of disgruntled drivers behind you. The next thing you know you are overtaken by one of the many dilapidated Land Rover Defenders in service in the vicinity with a couple of sheep in the back.

After a few minutes of struggling against the inevitable you give up, press the button again, the little green light goes out as ECO mode is cancelled and normal service is resumed.

While the ECO button is a well-meaning addition to the vehicle’s specifications, one cannot help but be but a little wary of fuel economy devices that require drivers to actually do something. Unless they own the van concerned and pay the fuel bills themselves, or are part of some sort of incentive scheme that rewards the most frugal driver in the fleet, or are at the mercy of an onboard behavioural monitoring device, odds are that they won’t.

So will the ECO button be pressed again? Maybe – if the regular driver remembers to do so. And it’s already starting to get dusty.