Hotel21 (Broocie) did this job on Kate's car at the cambelt party. When you have taken off the plenum etc. to expose the stat. the metal pipe running from it to the radiator hose runs through a tunnel under the cam housing, and is tough to get at. You have to pull this pipe out of its joint on the side of the stat. towards the near side, and if the joint is seized it's almost impossible to get enough purchase because of the confined space. At Shifnal it resisted brute force and every other form of persuasion and had to be smashed with a hammer and chisel, removed in pieces and discarded for a 2nd hand replacement.
I didn't see the re-assembly being done, but the end result, (the thermostat wasn't present in Kate's car, by the way), was engine temp. back to normal, and improved mpg and cabin heat.
A main dealer would no doubt charge a fortune so if you can do it yourself you'd get a better result at cheaper cost.