Update 13 August 2020:
I really didn't think I'd be adding to this thread, but the A/C fix my dealer made in July didn't fix things at all.
Last week, I noticed that no cold air was coming from the vents, just ambient-temp air, no matter what setting I dialled in to the controls. Deja vu - that's what was happening before.
It turns out that there is a fault in the A/C system. On Tuesday I went to the dealer who examined the engine bay with a UV light and sure enough, you could see the dye that they'd added to the gas when they regassed the system last month visibly splattered in the bay. So less than a month on from the regas, the gas had escaped from the system. The mechanics told me that the cause was a broken pipe from which the A/C gas had leaked.
Not sure which pipe; here's a photo of my engine bay: the pipe is one of the vertical ones on the left (I think) -
Fixing the problem requires a new part to be ordered and then a repair undertaken that will involve removal of the front section of the car to gain access to the engine bay to effect the repair. And this part has to come from Japan so we're looking at early September at the earliest for the repair, which will be under warranty.
Really disappointed with this turn of events. Looks like I have a new car with a 'Friday part'. Hoping the rest of the car is made with Saturday-Thursday parts!
Reflective thought - the dealer told me that the A/C hadn't been properly gassed at the factory before it was delivered. What if, in fact, that was done entirely properly but the pipe fault being present at that time meant all that gas leaked out. That's what it looks like to me.
So I wait for news from the dealer and, in the meantime, enjoy heatwave air π