zafira 2.0dti takes along time to get warm?

can anyone help. The heater on my Zafira 2.0 dti, 51 plate takes along time to get warm in the car. After about 4 to 5 miles I can start to feel heat coming through the vents and the temp gauge starts to rise slowly to normal. I was wondering if it could be thermostat problems also as the temp is showing cold can this cause the car to act as if the engine is cold when its actually running at normal temp. Can anyone thanks Andy