Rover 75 heating problem

I have a problem with my Rover 75 that the heating doesn't blow very hot. I suspect it is the matrix blocked as during the summer I had to put some rad weld in the water as there was a small leak. I know from expercience or other cars that it is allows best to have the heating running at full blast when you put in Rad weld and completely forgot!. I have tried to to reverse flush the system and even tried some rad flush! The first time I did it there was a slight improvement from now heat to a very tiny bit of heat on a hot day! I want to check that this isn't a red herring as I have also noticed the temp gauge no longer sits dead on the centre as it used it goes down slightly when driving at speed, and also there is absolutly no heat out of the passenager side and back! Sorry forgot to say it is dual zone climate controlled. so I was thinking maybe it's a sensor or something else? as also if I put it on auto it just turns the fans down to 1 even when it is very cold!!