Cleaning out Heater Matrix

Is their anything I can do to flush out the Heatewr Matrix in a UK 190E I Have used Rad Flush for three days then flushed out the system with a hose and the water initially came out brown but soon changed to clear
and appears to come out just as quick as going in
I have fitted a new Thermostat with the only limited effect, the heat from the input side was about a degree or so to the output side and the Garage said he could only think the Matrix is clogged, But I want to try anything before I take the Fascia out many hours

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