So I got my new thermostat, put things back together and was filling the system (with distilled water, in case it had a leak) when I noticed a drip. Closer inspection revealed a hairline crack in the expansion tank so, of course, I'm going to replace it. I did notice that the coolant level was quite low when I first started working on this overheating problem. I wasn't sure if the low level was due to the overheating causing the fluid to boil off, which I hadn't noticed happening, or it's having leaked out prior to the incident, which I also hadn't noticed. I guess the latter probably was the case. With hindsight I can see that I probably should have added fluid back to the system and looked for leaks before I sarted pulling the waterpump and thermostat.
I've been using the Bently manual as a guide for this car and, for the first time, have a problem with their directions. The manual says to remove the coolant level sensor from the bottom of the expansion tank by turnig it counterclockwise, but this is wrong! After I removed the s-shaped air duct that's located close below the ex-tank (a quick, no tool process), I was able to see that the sensor comes off by twisting it clockwise. I don't think it even has to be removed from the tank to remove the tank from the car. I also took the fan and shroud back off to better see what I was working on. I'm having trouble getting the tank free of the fluid connections on the bottom of the tank and wil probably just break the old tank up to enable me to have more wiggle/pry room. I've sprayed a small bit of silicone into the stuck joint and will wait a while to see if it frees up before I break up the old tank.