Climate control not heating

Hi, hope some one can help.

My 2004 Octavia 2L PDi is fitted with climate control, i have had on and off issues in hot weather when i have had to turn off the climate control after an hour as it had become ineffective  ... Not much fun when drining in Spain. It has been re gassed and pressure tested in the summer.

Between Christmas and new year on a gentle 130 mile run up the motorway i was starting to feel a tad chilli depsite the climate control set at what should be a very pleasant 22c. The climate control was not pumping any heat into the car.

Turned climate control off and the standard heater heated the care up very nicely.

Any idea what the problem is or how to eliminate each component to try and resolve the matter.

Thanks in advance.