Right, after the car barely started after work (despite being absolutely fine that morning), we decided to do some more digging.
We noticed that after disconnecting and reconnecting the battery, the drain was only ~100mA, which is closer to the expected value. So we turned on the ignition (didn't start engine) and then back off again, to watch the drain. After a little while at ~1400mA, it dropped to the 750-800mA it was doing last time.
It didn't seem to get any better, so we started on the fuses. Most didn't have any effect, but one fuse, no. 23, marked "Engine injection module/Rain sensor/Gear lever/Starter relay", when removed dropped the drain to just 70mA, which is spot on for what is expected. When replaced, drain remained at 70mA.
We can't think of any legitimate reason why any of the components that fuse is responsible for would have any reason to still be drawing power 20 mins after the key was removed.
But, I'm not an expert. If anyone here knows better, I'd love some advice. I'll be contacting the local dealer ASAP, and try and get one of the techies to talk to me and see if they agree with our findings.