Upcoming heatwave

Quick related question - what's the best thing to do with Jerry cans filled with fuel? OK left in or on the car in such hot weather?

Good question to which I don't know the true answer.
Any risk assessment would say store out of direct sunlight and outdoors.I certainly would not keep any inside the vehicle if it's parked and sealed, simply as if it did leak it's a big mess to deal with. Outside the vehicle not as worried.

I Guess you could open each to normalise the pressure at some point during the day, but I have no idea of the actual container pressure rating if it even has one.

I've had plastic petrol cans swell up to he almost round just in normal temperature so I am very cautious about leaving any in the car at all. Bearing in mind that the vapours are what explode if it has the slightest leak you basically have a large contained volume of combustible gas just waiting for an errant spark and game over.

Your wife's car will be just fine - I'm one of the folks in the tropics with a JB74 that lives outdoors, we're expected to hit around 36*C today.

To help me cope with the heat, my car is fitted with window tint on the sides & rear and with "vent visors" that allow me to leave the windows down a half inch or so, without concern for sudden rain storms or thieves.

Gas/petrol in a plastic jerry can, inside the car is a big NO, outside the car in the sun is still a NO - you're going to develop some surprisingly high pressures in the can due to expansion & evaporation.

What did the temperature end up at? here in Australia were getting reports that the UK has broken the 40⁰ number

It was 40 degrees in some parts (Cambridgeshire, Lincolnshire). 39 in the shade where I live in Essex, never felt heat like it in the UK, but thankfully not humid, so more bearable than somewhere like Florida.