It was eventually agreed that Mercedes, Ferrari and Red Bull would provide "mule cars", with 2015 models modified to replicate the high downforce expected next year.

The teams have three solo tests each, divided into sessions for wet running, soft/supersoft/ultrasoft, and hard/medium, followed by a 10th "final validation" test after the last race in Abu Dhabi where they will all run together.

A key stipulation from Pirelli was that the teams use their race drivers, or at least an experienced test driver – the sessions were not to be used for giving youngsters some extra mileage.

We are already at the halfway point in the programme, with five of the tests run thus far. And the fascinating aspect is that Ferrari is the only team to have used its own race drivers for the testing, with Sebastian Vettel having done both tests and Kimi Raikkonen one, with one day going to Ferrari-affiliated Esteban Gutierrez.

In contrast, Manor's Pascal Wehrlein has conducted the two Mercedes sessions by himself, and Sebastian Buemi ran the single test held so far with Red Bull.

So why is Vettel so keen to use up some of his precious time off? It's just another little sign of his "marginal gains" approach to racing, which sees him exploring every possible opportunity to seek an advantage.

2011 pay-off

This is the man who, after winning the 2010 world championship in Abu Dhabi, flew to Europe for Red Bull celebrations and then came straight back to Yas Island to conduct the first major Pirelli test.

He was also the only driver to visit the Italian company that winter, so keen was he to get to know F1's new tyre supplier, and find out what direction they were taking.

In 2011, the first season with the Italian rubber, he dominated the world championship, and generally left teammate Mark Webber far behind. Of course many other factors were at play, but there's no doubt that he adapted better to the new tyres.

While the current 2017 testing programme is blind, meaning that teams and drivers are not given details of constructions and so on that they are running, any mileage that could help a driver prepare for next year by giving him a feel for the tyres is surely useful.

And more specifically by taking part in the testing at an early stage a driver can to some degree try to direct development in a direction that suits his style.

"This is 100 percent sure," said Pirelli technical boss Mario Isola when I asked if that could happen. "I can sign here! Because if a driver likes a specific construction – they don't know what they are testing – but if they feel confidence in one prototype, they push to have this prototype. They don't know what we select, but they are pushing in their comments.

"Knowing Vettel for some years I have to say he is very interested in testing, he is very interested in understanding what he's testing, asking a lot of questions. Sometimes you have to stop him asking and say, 'You have to tell me, not ask questions.' But his personality is like this, so he is very interested in understanding..."

Still a benefit

Even without knowing exactly what they are running, good drivers can draw some conclusions.

"The drivers have a feeling," said Isola. "You cannot tell a driver who's driving a car that you don't have to feel anything, because it's very important for us also to have the comments and feedback from drivers. How much is this advantage is difficult to say.

"But they have a feeling. it's not next year's car, it's something in the middle between this year's car and next year's car.

"They don't know what they are testing. Sometimes they think it is a compound, while it is a construction, or it is a different profile. There is a lot of analysis that we do. If they say this prototype for me is fantastic, we need to understand why.

"Because maybe it is a construction, and because it is the same on all the compound levels, it is something we can consider, for sure. But then we also cross some specification on the different teams, to see if we have the same feedback.

"Sometimes it happens, sometimes it doesn't happen. If it doesn't happen, we need to understand why.

"The driver comment is very important, the data are very important, all the indoor [factory] tests that we are running before running on track are very important, so then we have a big amount of information that we need to put together to take the right decision."

Pirelli is clearly delighted to have Vettel on board and so keen to get involved, and there's no doubt that it's keen to see other established names take part.

"They are free to decide with the teams. We discussed in the past which drivers we wanted for this test campaign, and we agreed that our preference was on race drivers, or test drivers, or anyway drivers with some experience to give us also feedback, because changes are so big we need not only the telemetry, but also the feedback from the drivers.

"Then it's up to the team. For example Red Bull used Buemi because he is their test driver, and he's running a lot in the simulator.

"It's probably important for them to have a correlation between the simulator and the car. It's a decision. Buemi did a very good job in Mugello, it was the first test, but he did a good job.

"We didn't say that we are not happy with Pascal's test, he did a very good job. He's a race driver. We would like also to have feedback from the current Mercedes race drivers. I am sure that they will make a plan to have their race drivers at future tests."

In recent weeks both Lewis Hamilton and Nico Rosberg have been asked about whether they saw taking part in the Pirelli testing as potentially advantageous, and they both pretty much shrugged their shoulders and said if the team asks them to do it, they will.

Neither appeared likely to take the initiative in the way Vettel clearly has.

"I'll see, I'm not sure," Nico said at Spa. "We haven't spoken about the test that's coming up yet, so let's see. If it makes sense I'll do it, and if it doesn't, I won't."

Obviously they could both yet get involved in the remaining Mercedes solo test in Barcelona on October 12-13, and the group session after the last race, but the fact that they haven't actively campaigned to do so thus far is a surprise.

For Rosberg, whose approach is not unlike Vettel's, any effort to get a step ahead for next year would surely be worth it.

Meanwhile, the clock is ticking, and even if Hamilton and Rosberg attend that mid-October test they may have a limited opportunity to have a real influence before the 2017 specification is set.

The same applies to Daniel Ricciardo and Max Verstappen, for whom the next opportunity is October 14-16 in Abu Dhabi.

"We don't have a lot of time," says Isola. "Just after the tests we had in the last two weeks in Barcelona and Paul Ricard we had a meeting, immediately, the guys were working really day and night to prepare the analysis.

"We had a meeting, we analysed the results, to decide the new prototypes for the next group, because the next group is in a few week, so we need to give the factory the information to build the prototypes and so on. But this is the time we have, so we cannot do anything else than this."