I've just had this issue fixed on my car, I had it done at a local garage who had previously fixed a golf with the same issue. I was quoted £220 but in the end I had to pay £266 as I had a coolant pipe shear off that goes into the heater matrix. This happened 3 days before the car was due to get the heater matrix fitted so it wasn't such a bad time for it to happen. It's not a big job as others have said you can get to the heater matrix without removing the dash.
I also had the silicate bag removed from the expansion tank to stop this issue from happening again as it's the bag splitting that causes the heater matrix to block up in the first place.
Can you add in any more info on this "silicate bag" that is normally in the expansion tank - now that sounds like a very weird thing to have in there! I hoping that there is some slight unintended misinformation in that sentence, though something is clearly causing these heater matrix to get choked across this VAG platform.
Silicate is normally used to absorb moisture and as such it has no place inside the cooling system, pointless really as it would get wet quickly, then what. Now if it was placed inside the air box for the heater, it might have a short term benefit but it would need removing every day for drying again, so again - why fit it.