Not necessarily a heater matrix failure.
More likely to be the tap that hides behind the plastic cover on the passenger side just underneath the dash.
1) Push back passenger seat, stick your head under the dash with a torch, look for the plastic cover held on by two screws to left of centre console.
2) Remove two screws and you are now looking at the single piping coming from the engine bay to the heater matrix.
3) Problem may be the tap on the side of the matrix affixed to said piping.
4) See if coolant is leaking from here, move piping a little from side to side to see if drips come from here.
5) If so just order a replacment tap from the dealer - only £15 max.
6) Fairly easy job to do, couple of fiddly screws, that's it.
Problem why this issue of the tap leaking is so common is that the plastic cover puts stress on both the piping and tap (it's a tight fit, which means the piping is pushed to one side, putting stress on the fragile plastic tap). Solution is not to put the plastic cover back, you don't really need it.
This is a common problem on the 916 TS Alfas, do a search on here and a few threads will come up. The SW Rep did a photo guide in one thread of the tap concerned.
If no problem from the tap at all, then yes the heater matrix itself is the next step. But I bet you it's the silly little £15 tap!!
Hope that helps.
Last edited by Spider95; 28-01-10 at 20:00.