No experience of alfas but is the rad/ engine warm? Can you disconnect the hose linking the matrix to the coolant system from the engine bay and feel if it's firstly warm and secondly flowing? If it's all hot then is the mechanism- usually a rod activating the flap/ mechanism which allows warm water to enter the matrix? Only had to change one heater matrix in over two hundred cars. It's usually a mechanical issue or a fan problem.
If the thermostat etc checks out OK then the likely culprit is the valve that controls flow to the matrix under the dash.
You can change it in situ, without getting the dash out. I did one of mine, and followed advice from SpiderS4 to take the drivers seat out first to make access lying on my back easier. Good advice! It is a bit fiddly but made a huge difference.
You could also check that the slider control is actually connected to the valve before you go any further. The connection is a very stiff wire from the slider to the valve.
as Anglo says, a job you can do yourself, you need to be a contortionist and factor in a couple of hours on your back.
The heater valve is above the pedals, making life even more difficult
here are some simple instructions how I did mine.
1.Remove left hand seat to give you room to lay on your back.
2.Remove plastic air duct on heater (2 small bolts, duct slides off, hang it out of the way)
3.Now you have a view of the valve up there (you can use a mirror on the floor and a torch to get an idea what you are up against, before tackling it!
4.Tip: I also remove heater hoses in engine: one off the water pump and, one off the top of the intake plenum – this way you will drain some of the water in the heater so it doesn’t pour out inside the car. Blow through the pipes to clear the rest. I used a spare length of hose, blocked on one end, to quickly slip over the waterpump outlet, so you don't lose too much antifreeze.
5.Undo hose jubilee clip on heater valve, then slide off hose (you can pull somewhat from the engine bay side to help it. I spray some silicone spray where it goes thru the bulkhead so it allows movement from inside the car)……Tip: you want to pull it off just far enough so it releases, as putting it back is also a pita!!)
6.If possible mark the control wire that activates the valve, or at least note how much is poking through, so it goes back roughly how it came off. Then loosen the bolt holding the wire.
7.lay some rags on the carpet to catch any water and remove 2 bolts holding the valve to the heater element, pull off valve and unbolt and slide off the control wire. Make sure the rubber oval seal comes off too. Replace this seal (you will need to order that separately)
8.Replace in reverse order, drink a few beers, and cuss the Italian mechanics who decided to move the valve to the heater, when earlier alfas had it under the bonnet, and you could change it in 5 minutes!
one of those little led lights you strap on your head also helps