How can I fix my heating in a 1999 Citroën xsara westcoast?

I have a 1999 Citroën xsara westcoast 1.4 and for the past 3 months I've had no hot air come through my blowers inside. The blower works fine but no hot air comes through. I've had it flushed through and that was fine, I've had the thermostat replaced and several guys have looked at it for me. General opinion is that I'll need a new heater matrix to cure my problem. Had quotes to replace with new but very expensive. I am a complete novice with cars but am willing to try and sort the problem out myself. Does anyone have any info that can help? Is this the problem I have? Is it much of a job to replace and can I do it myself? If not the matrix what could it be? Any help would be great. Cheers.