Thermostat is located on the number 24 where the temp sensor is. Simply fallow the big pipe from the radiator to it. Its simple to change, 3-4 screws and jubilee clips, but you must went the system afterward.
Its on the right hand side of the engine block, with 4 or 5 black coolant pipes going into/out of it. As Sven said, dead easy to change, just make sure you have a handful of new jubilee clips. Also, you'll drain a couple of litres of coolant doing it, so you may as well just replace it all with new stuff anyway.
This is the sort of thing that you're looking for.
Picture courtesy of shop4parts which is also one of the cheapest places you can buy a new thermostat. (£43.68 for a 1.6 thermostat delivered)
Buzz
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& now ex owner of a Bravo 1.8