After the engine has been running there is steam coming from underneath the bonnet. The steam is white in colour.
On inspection there is coolant fluid around the heater matrix, often this is concentrated near the pipes entering the matrix.
The air coming from the vents is not as warm as it was previously, even with the heater controls turned up it still doesn't get as hot as it did.
The heat level coming from the vents stays around the same temperature, it doesn't get any warmer even after the heater controls have been turned up.