Thanks for the advice, Burnerman! It turns out that the kill switch won't fit my battery anyway - the battery posts are too small for the kill switch clamp to go on, and the post that comes with the kill switch is too large for my car's battery cable connector thing. So I didn't use it.
But what I did discover was that there was a pair of fuses that I had missed when I checked the fuse boxes! They are shown in the photo below. I pulled out the yellow plastic 'thing' which holds both the fuses in place, and then tested the current between the negative battery post and the post cable connector (not sure what you call it), and it was zero! I put the two fuses back in, and it was 0.3A! I took out the 15A fuse, and it was still 0.3A, then I put the 15A one back in, and removed the 10A one, and it went down to zero amps, so I took it out completely.
I then watched a video which explains what all the fuses are, and 'R/LP' stands for 'Room Lamp', which I presume is the internal light on the ceiling of the car. That would explain why it took 0.3A for 60 seconds and then turned off, last time I checked with the multimeter. But it must be coming back on for some reason (possibly a dodgy door switch? Or a dodgy relay?) I haven't actually been back to look at the car to see if the ceiling lamp is, in fact, off - because I didn't know what 'R/LP' meant, and had to come back into the house to search on the internet.
So that's where I stand at the moment - I've removed the 10A fuse permanently, but I haven't connected the battery back yet - I will do that tomorrow when I actually drive the car, because I still don't trust the damn thing!
