...this seems like a good idea.
This was just shy of £25 on Amazon, and while I doubt I will need to use it
because I totally trust this car to not set on fire,
there's no reason I should not have one handy. £25 and a tiny bit less boot space is a small sacrifice for not losing four years of work.
Also...the boot, where the extinguisher is bolted in, was not part of my interior cleanup rampage, and it looks absolutely terrible. The carpet is a Mazda carpet, but it is not from a Mazda 323; it has a nicely-sewn Mazda logo suggesting it is a carpet made for a Mazda Something but it has been cut about to fit inside the back of the GTX (which is why I didn't mind drilling a hole through it). It is also hopelessly warped by the car not being watertight when I bought it. I'll look into my options.