Then you need to
1. Check coolant is full and bleed properly (including those little Renault bleed points with plastic caps) Check feed pipe is getting hot to heater matrix.
2. Check it is NOT a water valve control heater (in other words .. an air blend heater) and look at the heater body cable control, it will probably be that. If you think it's water valve heater, you need to look at this valve. This can be electronic (like BMW) or manual valve.
3. If you CANNOT achieve hot flow to the heater matrix, it will probably be a blocked water offtake on the head or manifold. Sometimes quite small orifice. This is unlikely, but I have seen it before.