Does the hot water flow through the heater matrix all the time, or only when the dial is turned to hot?
I remember that on my old Bedford van, the temperature dial was connected to a valve, but there doesn't appear to be a valve on the new matrix that I've bought. I wanted to try flushing the system out first before going through the horrendous task of changing the heater matrix.
I notice that it goes cold immediately when you turn the dial to cold, which suggests the air is being diverted around the matrix instead of water flow being shut off.
The rad cleaner I've got in at the moment doesn't seem to be producing much of a result - I thought it would have worked though as the air was coming out mildly warm, suggesting a blockage, though the pipes seem quite hot under the hood where they come out and go past the engine. I guess there's likely to be a drastic difference in percieved temperature though, the air isn't going to be scalding even if the pipes are. :)