Are you 100% sure it is the heater matrix 
Well what els could it be? Temp read bang on 90 all he time and don't take long to get up to temp.
Ran hosepipe through heater matrix pipes and it flows fine. Both pipe to and from the matrix get nice and hot. Had a good fiddle around inside the car and all the heater dials and switches seem to be working ok.
Plus if the heater is off for a wile then I turn it on. I get hot ish air come out but it fades back to cold as if it get hot ish but the blower is taking the heat from it and cooling it down to much. Wich tells me it's got hot water through it but for some reason it's not staying hot enough for the heat transfer.
What type of matrix is it? Is it just a coiled ish pipe that has fins on it? Or like somebody said in another thread is it a 2 chamber box type thingy? Anybody got a diagram of a cut in half one?