Disco 3 (LR3) Heater issue

This is my second D3, my first having 200k miles was still going strong but a commercial model and I needed some rear seats, so with 165k on it and apart from knocking antiroll bar bushes the new car is a lovely drive but the heater is useless. With the engine at working temperature the heat out of the driver side vents is just about acceptable but the passenger vent is tepid air at best. Turning off the driver vents makes no improvement to the other sides performance. The heater control will adjust the heat from freezing to tepid very smoothly, there just doesn't seem to be much heat in the matrix. Can the system be flushed out and cleaned?
Cheers Ian