206 SW 2005 Petrol Low temperature on temp gauge after change of timimg belt

[Briten]

Hi. I had timing belt, drive belt and water pump exchanged on my 206 SW Petrol Year 2005 one week ago in a garage.
Before the exchange the watertemp gauge went up to about half of the scale, as it should do.
The engine works fine but one problem has occured after the job.
The watertemp gauge is not showing correct temperature. In fact it doesn't move at all.

When I put on the ignition the needle in the gauge moves up a small distance, up to 0 in the scale.
Then the motor starts and I drive off. Before, it took 5-10 minutes for the needle to reach half of the scale.
Now it doesn't move up at all.

BUT I get warm air into the compartment, feels ok. Don't know about the temp inside the engine.

Went back to the garage and told them about the problem. They couldn't give me an answer.
They talked about that the temp sensor or the thermostat could be faulty.
I find that not likely, it worked ok before the exchange and why should it break down just because of exchanging those parts?

My questions are:

Is there a way to check this? Can I do something with the car to solve this?
Could it be that the car (the ECU?) needs some time to readjust its settings before showing correct value?
(The coolant has also been completely renewed if that could be a reason for the problem)

[mickthemaverick]

You may have an airlock around the thermostat so I would bleed the cooling system thorougly using a high funnel type kit and see if you get any air out. There are videos on Youtube showing the method, this is one: