Meanwhile, Patrick's got some technical analysis on what's going wrong at Williams.

Patrick Beames: I think I've worked out what Williams' problem is - they're running on square wheels.

Andrew Benson
BBC Sport’s chief F1 writer

Valtteri Bottas said Mercedes needed an upgrade to solve the inconsistent balance problems they had at the first test, and, boy, do they have one. The car has turned up in Barcelona with a spectacular-looking revamp on the nose and front wing. It exacerbates the differences in philosophy between the world champions and Ferrari.

In essence, Mercedes have gone further down the route they were on before - with a front wing that has flaps that rise up from the centre towards the end-plates. This not only maximises the downforce the wing can create, but also directs air from the flaps around the outside of the front tyre - a new end-plate emphasising this on the Mercedes by turning outwards, rather than inwards as before.

The Ferrari, by contrast, has flaps that slope down from the inside to the outside, the idea being to direct air inboard of the front wheels, aiming for aerodynamic consistency, but also limiting the amount of total downforce the wing can create.

Mercedes and Red Bull have taken one extreme, Ferrari and Alfa Romeo another. And the other teams are all somewhere on the line in between. Who will be proved right?

The front/nose is not all - the car has new vanes and fins in the barge board area in front of the sidepods, and a series of revised aerodynamic shapers - or 'slats' as they are known - on the floor. And the sidepods themselves are new, too, more compact, creating more space for the airflow rearwards. It's basically a B-spec car already.

This is how Lewis Hamilton's car is looking today. The technically minded among you will notice some changes.

Time for me to hand over to someone much cleverer than me. Here's Andrew to explain more...

Image source, Getty Images

Lewis Hamilton has shaved his best time down to 1:20.764, but that is still only keeping him in fifth place.

Hamilton's time has come on the C2 tyres, you know, the hard, but not that hard, ones.

Andrew Benson
BBC Sport’s chief F1 writer

Williams had, to say the least, a difficult first test, the car not running until the penultimate afternoon, and George Russell and Robert Kubica managing only a handful of laps each.

Russell emphasised at the end of last week just how much Williams were now up against it: “We have a lot of work to do to make sure the car is in the best-possible state with no issues and that we can get the laps that we need as we have some catching up to do.”

As for how the car felt on the Formula 2 champion’s first experience of it, Russell said: “It was quite nice, it felt stable and I had a good feeling from it but I think there is a counterargument as I am not fully on the limit, I don’t know the limit of the car, so I wasn’t able to find what that is yet.

"Until I fully wring everything out of it yet it is going to be difficult to know where we are at or the full limitations or where we need to improve.”

Image source, Getty Images

Want some more Williams info? Here's our man Andrew Benson...

Craig: Week 2 of #testing, external is here! Finally we’ll see the teams showing a bit more pace and Williams and McLaren will lead the field right? Please?!

Er, Craig, bad news I'm afraid. We've now had all 10 drivers set a time and the slowest two are McLaren's Lando Norris with a 1:22.306 in ninth and Williams' George Russell last with a 1:44.535. So only 26 seconds for Williams to find to move to the top. No problem.

Andrew Benson
BBC Sport’s chief F1 writer

Throughout the first test, the new Mercedes looked difficult to drive, and at a sponsor event at the end of the week, Valtteri Bottas expanded on the problems the team had been having. ”We have work to do, but it's a good feeling in the sense that it feels like there is potential," said the Finn. "We do need to make improvements.

“We were struggling a bit over the week to find a good balance for all the corners. There were some corners that were good, some corners we would have balance issues and other corners [where] we would have massive balance issues.

"We were getting to a much better state at the end of the week, but some of the handling issues can only be sorted with some upgrades. We're hoping to fix those hopefully soon.”

Image source, PA

By the way, Pierre Gasly's leading effort so far today is a 1:18.647 on the yellow tyres (the C3 medium compound)

Remember the rainbow tyres of 2018? Ah, glory days.

Now we're down to five different sets of tyres being used in testing. Two white ones (they're the hardest ones - the one with no 'brackets' is the hardest of the lot), a yellow one (that's a medium compound) and two red (they're the softest, with the no brackets option being the softest possible.)

Got all that? Good.

Image source, Pirelli

Some movement on the times and Lance Stroll has zoomed himself up to second with a 1:19.139, with Red Bull's Pierre Gasly leading the way thanks to a 1:18.647.

It's a big year for the Canadian Stroll as he tries to convince the Formula 1 world he is deserving of his seat at Racing Point, after his dad led the takeover of Force India last season.

Stroll, 21, only picked up six points in 2018 - four of them in one race when he came eighth in Azerbaijan and two for a ninth-placed finish in Italy.

Mark's on a boat (see 08:54), Thorpey's chilling in the sun. Alright for some.

Image source, Thorpey

Thorpey's not the only one one missing the great man out there. Will Alonso miss F1 so much he wants to come back next year? Who knows?

Andrew Benson
BBC Sport’s chief F1 writer

A man on whom the spotlight is very much shining brightly at the start of this season is new Ferrari recruit Charles Leclerc, and so far things have got off to a good start, with the fastest time of the first test. Many are licking their lips at the prospect of Leclerc v his four-time champion team-mate Sebastian Vettel. But Leclerc says all is harmonious within the Ferrari camp, and he is emphasising his status as the junior partner.

“We are working very well together,” Leclerc said. “Seb is a very nice guy, not very difficult to get on with him so that’s good. We’re working on different things with the car. I won’t go too much into details but it has been good, and it is also good to have Seb next to me during these days, even though it’s quite difficult to compare one day with the other. It’s quite good to see what are his strong points and what I can learn from him. It has been interesting in many senses.

Leclerc said he still had a fair bit to learn in terms of how to best make use of all the team’s resources, and felt he needed to improve “in terms of feedback”. “Seb is incredibly strong at that with very big technical knowledge, so that I can definitely improve.”

Image source, Getty Images

1. Gasly
2. Magnussen
3. Leclerc
4. Hulkenberg
5. Hamilton
6. Albon
7. Giovinazzi
8. Stroll
9. Russell

No time yet from Lando Norris

Speaking of Ferrari, a nice picture of Charles Leclerc for you. He's third fastest at the moment today, but has only had six laps on the track, compared to 12 from Pierre Gasly (fastest), 15 from Kevin Magnussen (second fastest), 20 from Nico Hulkenberg (fourth fastest) and 18 from Lewis Hamilton (fifth fastest).

Andrew Benson
BBC Sport’s chief F1 writer

They say it’s impossible to predict form from pre-season testing, and yet after last week’s first test everyone agreed on one thing - Ferrari were ahead of the field.

The SF90 appeared to behave benignly for its drivers and was very fast. Had the first race been held then, they would have been overwhelming favourites for it. But the first race is still more than two weeks away, and in between then and now are four further days of testing, at which many teams will have new parts, and time to fly out yet more upgrades to Melbourne.

No-one is writing off Mercedes yet, but there is no doubt the world champions could do with showing a bit more pace this week.

Image source, Getty Images

Renault's Nico Hulkenberg was fastest on the final day. The German was split from team-mate Daniel Ricciardo in third by Toro Rosso's British-born Thai rookie Alexander Albon, with Mercedes' Valtteri Bottas fourth.

Hulkenberg set his fastest time on the softest tyres - which were used by all of the top four - before stopping out on track late in the day.

Bottas and Hamilton, in fifth on the second-softest tyres, appeared towards the top of the times for the first time all week, ahead of Leclerc and British rookie Lando Norris in the McLaren.

Williams, whose car only made it on to the track on Wednesday afternoon - midway through the third day of the test, concentrated on learning their car and drivers George Russell, another Briton making his debut this season, and Robert Kubica propped up the times.

Meanwhile... Mark's OK.

Image source, Mark Philpott

Chris Medland
F1 journalist in Barcelona

Temperatures are just 9C at the moment here in Catalonia, and that leads to aerodynamic testing being the main focus while tyres are struggling in the cold. Renault says its morning program with Nico Hulkenberg features "a couple of new aero parts to try out and will focus on longer-based runs and setup work."

Once the sun climbs then so do the temperatures, and the best window for testing is in the middle of the day. In a few hours it should be close to 20C, which is nearly as hot as was registered in Wales yesterday. Who knew the teams should have gone testing at Pembrey?

Image source, Getty Images