100 miles from 20 quid isn't too bad. It's about average. The stat deffo won't help as the car will be running cool, so the coolant temp sensor gives the ecu the signal to dump fuel in, which is how it works when the engine is cold. The stat is a 20 minute job, remove pas pump and belt and the stat housing is right there with the 2 10mm I think, screws looking right at you. Make sure you fit the stat the right way round. As I said, 20 minute job. Remember you have to refilled the entire cooling system after this job, and while the stat is out, I'd flush the complete system with a garden hose, as the only proper way is by removing the stat.