From Rave:
"Thermostat - Main valve
The thermostat is closed at temperatures below approximately 82C (179F). When the coolant temperature reaches approximately 82C the thermostat starts to open and is fully open at approximately 96C (204F)."
However there is also the By-pass flow valve:
"The by-pass flow valve is held closed by a light spring. It operates to further aid heater warm-up. When the main valve is closed and the engine speed is below 1500 rev/min, the coolant pump does not produce sufficient flow and pressure to open the valve. In this condition the valve prevents coolant circulating through the by-pass circuit and forces the coolant through the heater matrix only. This provides a higher flow of warm coolant through the heater matrix to improve passenger comfort in cold conditions.
When the engine speed increases above 1500 rev/min the coolant pump produces a greater flow and pressure than the heater circuit can take. The pressure acts on the flow valve and overcomes the valve spring pressure, opening the valve and limiting the pressure in the heater circuit. The valve modulates to provide maximum coolant flow through the heater matrix and yet allowing excess coolant to flow into the by-pass circuit to provide the engines cooling needs at higher engine rev/min."
Hope this helps - I guess the key question is did you drive the car for a good run at reasonably high engine revs?