Heater problems

My heater only seems to blow out hot air. The controls move perfectly but obviously aren't moving something they should.
Any suggestions what I should check or replace?

1990 Cabby

Get someone to check in the bay (to the left hand side of the servo) while you move the slider that the valve for the heater is moving..





Quite often seizes… about 6 or 7 quid on ebay..