I've finally fitted a kill switch. It was a bit difficult because the negative battery terminal lead from the car is only just long enough to reach the negative post of the battery, and couldn't reach the negative post of the kill switch, until I put the whole lot down the back of the battery.
Also, as the Picanto uses a T3 (Japanese) style battery, with much smaller posts than T1 (all other types of battery), I had to cut the negative post off one of the three flat batteries I've accumulated since the parasitic drain started, and then replace the post that comes with the kill switch, with the cut off post.
I had to put the cut off post in a clamp, then drill a 4.5mm hole in it (it's lead, so easy to drill), and then screw the bolt that came with the kill switch into it. I started the thread with about two turns, then took the screw out, then put the bolt through the copper rod that sticks out of the kill switch, and then into the post - because once I'd got the bolt all the way in, I didn't want to have to take it out, as it would have been cutting its own thread. Luckily it worked first time, and is rock solid - although it looks a bit Heath Robinson.
I bought two brass T3 to T1 battery post adaptors, and obviously only used the one for the negative post (I didn't realise that battery posts on a battery are two different sizes, positive and negative, to help prevent accidentally putting the wrong clamp on. Chance would be a fine thing with my Picanto, there isn't a millimetre of give in the negative battery cable from the car.)
I put the shim on, and initially put too much Vaseline on, which meant that as I tightened up the clamp from the kill switch, it rose up the battery post and eventually was loose. I cleaned off the Vaseline and put the clamp back on, and tapped it down further onto the post with a spanner, before tightening it up. This time it was rock solid.
After I attached the car negative clamp to the cut off post that I had bolted to the kill switch, I had to put the kill switch itself down behind the battery, because there was no spare negative lead to pull it up on top of the battery - which is where it is supposed to be. It comes with a double sided sticky pad, so you can stick it to the top of the battery.
Anyway, I then put the bubble wrap back which the kill switch came in, completely around the kill switch and the terminals, just to keep them from touching anything - if both terminals on the kill switch touched some metal, it would make the circuit and then my battery would probably drain again, because of the parasitic load.
The kill switch works perfectly with the remote control, it shows 12.4V on its LED display, with the engine off, and 14.4V (I think) with the engine on. It came with two remote controls, and they are really high quality, they have a nice feel to them, and look really well made, and work perfectly. I took the batteries out of the spare remote control, while I keep it in my house, and noticed that it has two CR2016 batteries, on top of each other, so presumably it needs 6V, not 3V.
After using the non-spare remote control to turn off the battery connection, I checked the car keyfob no longer opened the doors, and it didn't. I also checked the kill switch several times by putting the car headlights on, and turned them on and off with the kill switch. The car started fine, so the kill switch can obviously cope with the current needed to do that.
So at the moment, while I haven't fixed the parasitic drain problem, I no longer have to worry about it.