!! heater problem !!

An annoying problem with my heaters, engine temperature is fine but i get stone cold air though my heaters so im freezing in this weather.

Any ideas hope its nothing serious please help.

Maybe air lock in the heater matrix system, or a blocked matrix.

how do i sort this problem?

blast water through the matrix to unblock it if its blocked?

there is no air locks.

The control cable could have come off, does the temperature knob/slider move really easily?

EDIT, if the matrix is blocked you may aswell just replace it.

is the matrix easy to replace and how do i do it?

im quite handy wit a spanner so just need to know the hard bits lol

thanks for all the help!!