I had problems with my matrix being blocked, I could only get a dribble of water out of it when I tried flushing with a garden hose (hose tap open full).
The following is what I did to unblock it:
1. disconnect the heater bayonet, as described in Spirochete's post (beginning of this thread)
2. tried flushing with water, only a dribble of water came out.
3. used some coolant flush, the sort of stuff you can buy from Halfords. Took car for a run (about 30miles) to get car hot. Then flushed coolant again with water. Got more than a dribble this time, but still not a very good flow.
4. used some more coolant flash from Halfords, this time put 2 cans in and left them in for 2 days (did about 50miles). Then flushed with water again. The flow of water is getting better, but still not good enough.
5. Flushed the coolant with "Patio cleaner" as described in Spirochete's post (beginning of this thread). The only difference was that I used clear pipes in and out of the matrix and slowly fed the hole 4L bottle of "Patio cleaner" through the matrix. See photos.
6. To flush the "patio cleaner" out i changed the inlet pipe (the one that has the funnel on it) with a garden hose. Lots of brown dirt could be seen flowing out of the matrix and into the bucket. When the bucket was full I changed the pipes over so the water was going in the bottom and out the top. I did this about 12-15 time every time the bucket was full change the pipes, and each time more and more brown dirt came out.
7. When I was happy that all the brown dirt had been flushed I disconnected the outlet pipe to see that the flow of water was a lot better than it has ever been. I then reconnected the bayonet and filled the coolant up.
Now i get hot air out the air vents for about 20seconds then it turns to warm air. I know this isn't perfect but it is a lot better that I have ever had it before. I will flush the coolant again in about 1 month just to see if any more dirt has come loose in the matrix now that hot water is flowing through it.
Click on photos to make bigger.
I will be happy to answer any questions anybody may have.