fiat uno start

I've had exactly the same problem on a citroen AX.

Remove both pipes to the heater matrix.

Take a hose pipe and push this down one of the pipes leading to the heater matrix hold your hand around so most of the water is forced into the matrix. You should see rusty water come out of the other hose. Repeat in the other hose and then repeat the above a couple of times.

You should find this will sort the problem out. Check when you refill the coolant system that you don't get air locks although I'm not sure if this a problem on this sort of car. It could be that the heater control isn't opening the valve to the matrix just check that as well could be the obvious.

thanks David I think the control knob is ok it feels ok when turned and it does stop the flow when in the off position earlier today I ran the engine till it was hot then I felt both matrix hoses, the inlet wasn't very hot and the outlet was only warm, the main radiator pipes were both very hot. I borrowed a 30 foot hose pipe tonight so in the morning am going to blast the matrix hoses as you say, from both ends, it has to be easier than removing the matrix itself,

just out of interest, does anyone know, is mains water pressure higher then a cars coolant system pressure?

Yes it will be. But the flow of water will be stronger which should clear the gunk.