Bet the coolant has never been changed in the 15 years since your car was made. I had this with my old M reg Corsa. Drained the old coolant out, flushed it with a garden hose to remove as much gunge from the radiator and engine block as I could, then refilled with the correct mix of the specified coolant. Cured my problem because my thermostat wasn't stuck closed.
Chicks Fan
March 2009