The silly plastic plug at the bottom of the rad is a known real weak point, overtighten it and the head comes off, leaving the hollow screw body in the rad and your coolent all over the road. Been there done that! but it didn't go imediately, it went a couple of hours later in Tesco's car park....thank god I always carry a cheapy pound shop pack of that epoxy putty stuff.... 1/2 hour later back on road with a quick top up.
I've now replaced it with a brass one. To fully drain the system you need to do the other plug on the exhaust side of the block, as the rad one only drains roughly 1/2 water, I found about the same amount again came out went I did the block one.
Apparently the plugs on the diffs are the same, so got a couple of extras to do them too, but haven't got round to it yet.