I have a rover 400 and the heater is constantly blowing cold air how do I correct this?

The heater core needs attention. It could be clogged or needs replaced. The hoses also need to be looked at to be sure they are not plugged up either.

Heater core could be plugged as Twinkie pointed out; could also be a bad thermostat or control door inside of the dash.

Thanks, turned out to be an air lock!! Sorted now. Thatnks for your time

Mike