Old post this, but thought I'd add my experience.
Yes Mort, it sounds like the heater matrix. If you get down into the footwell (passenger side in Europe), look up at the back of the heater control console you should see a couple of pipes going into into it. Have a feel around these pipes and see if it feels damp. Also check the lower part of the heater console. If there are some drips or it feels wet it's almost definitely a heater matrix problem.
It's not too awkward to replace actually, and can be done without removing any of the dashboard. It's a little fiddly, but can be done.
From what I remember from doing it on my mk1 years ago, you just need to disconnect the two coolant pipes and also a couple of screws (or nuts, can't remember!) holding the matrix into place. Bear in mind some coolant will leak out when you do this.
Then you can simply withdraw the matrix out sideways, though it may need a bit of 'jiggling' to get it out.
Replacement is simply a reversal of this process. And you can probably pick up a second hand matrix from a scrapyard for next to nothing.
Oh, and if anyone is thinking of changing a heater matrix on a Peugeot 306, DON'T! It's one hell of a job needing the removal of the ENTIRE dashboard. It took me and a friend two days to do that job. Never again. Makes changing the Uno's matrix seem like childplay in comparison.