No heating

[Lucazade 6]

I barely get any heat from the airconditioning. 

I have changed the coolant (it was clean)

Checked the heather matrix, both from the firewall and in the footwell - it gets hot

Checked the flaps’ motors (they all work)

Checked the air mix flap itself (it moves)

Checked the air temperature sensor under the steering wheel (it works)

Yet when I put the heating on, even at max temp I only get some mildly warm air, whether the aircon is on or off. 

Has anybody got the AC unit apart to show me how it works or has any idea what else I could look at?

Thanks