Vauxhall Corsa heater blowing cold

have you tryed swapping the matrix pipes round if this doesnt work then do the heater controls work properly ie turn the dial to hot does the lever arm function ok linking to the matrix housing

I haven't tried swapping the pipes around but i guess thats an option. I have checked all the dials are working correctly as when you turn it to cold it does get a lot colder and when in the warm position you can just about tell it is trying to blow warm but not quite. I would be interested to know if there is some sort of a vale within the heater matrix that could be blocked. After flushing it out both pipes to and from the matrix appeared to be hot but the cold return cold be backing up from the engine. Any other suggestions anyone?