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Where is this beasty? If it's where I think it is (top radiator hose on the engine) then how do you get at it?

Beasty is the right word. It is located just above the power steering pump and is very difficult to get to with the engine in the car.
If the car is on a hoist, you may be able to see it from underneath, but the steering rack, cross member and steering pump are all obstacles.
So, it is very difficult to get to without removing the engine. Are you sure you have a problem with the thermostat?
The only picture we could find on the internet is in image 5 in this link .

Well hopefully not!! Apparently the thermostat was replaced(a second hand one) a few years ago, didn't cost much as I recall so I thought it might be an easy job to check if it was working ok. I've had several problems with the cooling system - overheating mainly. It was leaking badly, replaced the radiator hoses and even managed to extract the radiator itself(no mean feat I can tell you) and cleaned it out. It works much better now, no leaks, but it still heats up to nearly the halfway mark on the gauge, I don't know if that's normal or not. I was running it on water and replaced it with coolant and it still runs at the same temperature. Just thought I could test the thermostat but not if it's that tricky to get it out. I'm hoping that maybe because it's a V6 that it runs at a higher temperature? My wife has a Honda Odyssey and it's great - the temperature gauge barely moves off the lowest mark, so I was hoping I could get this Lantis to do that as well. Do you know if the Mazda is designed to run at that temperature? Thanks for your help, much appreciated.

Running in the middle of the gauge sounds right. If the thermostat was jammed shut, the car would overheat through lack of circulation and gauge running at the top. It doesn't sound like you have a problem.
Incidentally, it sounds like your wife's Odyssey might be running too cold.

Agreed. I would be worried about the Hondas thermostat! It sounds like it is stuck "open". The big thing you will find with this is the car will run rich and use morre fuel.

Halfway on the gauge is usually operating temp.

You are lucky your Honda even has a temp. gauge our Captiva 5 hasn't got one.

A lot of modern cars dont. Lets face it, how many people actually look at it. It will have a warning light come on when it over heats.

I constantly look at the gauge, I always make sure I'm as aware as I can be about how the car is running while I'm driving