It looks likes you have a leaking heater matrix, I'm afraid. A common fault on these. You could try radweld in the coolant, but more likely you'll have to replace it which is a dash apart job.

They new what they were doing when they sold this car,take it back to were it came and ask either they repair it or your money back.I bet they put two tins of radweld in to sell it.

The work involved is horrendous and very expensive it was the death of my fathers Volvo.