The thermostat retainers do have a habit of corroding into place. The best thing to do is :
1. drill and tap 2 M6 threads in the aluminium retainer, about 12mm deep,
2. wind in a couple of bolts
3. use a pry bar on the 2 bolts to twist the retainer free

If you do it this way, you'll never struggle again, and the 2 tapped holes won't cause any issues at all