I know it's a silly question. But the lever below the air con knobs has a straight thru symbol on the left and a recirculate symbol on the right. What difference does it make?

The 'straight through' or fresh-air function allows outside air into the ventilation system to flow from the vents; the recirculate function closes off the fresh air inlets, allowing only air already inside the vehicle to circulate

Pulling that lever on recirculate means that pre cooled air inside the car is sucked in under the dash to be cooled again. Helps the aircon to cool the inside of the car if the outside air is like 40 degrees.
I generally start the car, crack a window open to let some of the hot air out, then have that lever on flow-through. Fan on speed = slow.
Once the A/C starts pumping coolish air, up goes the window and I push the lever across to recirculate.
Fan on speed = maximum.
Takes a while to get comfortable inside a heat soaked car, though. Back off the fan (or not) as required.

On a hot day if you have a wagon one trick is to take a few minutes wind down the front windows and open up the tailgate to let the hot air out.

I've been puttering around in my TJ today - if I'm in a 50 zone, I keep it in second gear, 60 kph in third.
Keeping the RPM up helps the A/C a lot.

Global warming - it's all you guys driving your cars around all day and burning fossil fuels that has done it!

I always use the fresh air setting. Main reason is that if you have a leak in the exhaust system, it can creep in (through the rear of the car typically) and gas you. A lot of people use recycle, and in winter they have terrible fogging of the windows because the moisture in the air you exhale doesn't have a chance to leave the vehicle. Fresh is best.

Fresh is best.

Until you are following a cattle train, driving through a fire-affected area, or a Mushroom farm...

Until you are following a cattle train, driving through a fire-affected area, or a Mushroom farm...

Or sit behind any Diesel. Buses especially, I get a whiff of Chlorine! How or why, donno.

Or do a Top Gear episode where you drive through the town of Chernobyl.

I was going to say to go to recycle if you see someone coming towards you throwing heaps of dust, because that 'seals off' the incoming air (+ dust/smells/poisonetc). Also if you are going through a tunnel, maybe it is good to go to recycle. That is plain obvious. I still say Fresh is best (and that applies 98% of the time).

Having said this, there have been times up North when I have had to go to recycle, simply because the A/C could not cope with the load. That was in my NL Pajero. Our TF sedan had an A/C system which was fantastic under all conditions.