Heater problem with my compact

[powelly]

I just bought a 316i compact M43B19, all going well except a slight heater fault, at idle it's cold and only seems to get hot when under load or revved, then you get hot air. The engine warms up quickly, like within 2 miles, the temp guage then sits almost bang on half way, no fluctuating or anything nasty, no over heating, I drove it 37 miles in to work today, a mixture stop start, motorway and country lanes, the guage stayed virtually bang on the middle, drives really well, it's really just this heater issue. I don't think there are any head gasket problems, no mayo on the dipstick or under the rocker cover, anyone got any idea, I did read a few things that suggest the "duo Valve", what is this where is it??? Suggestions have also been made about the water pump, to be honest i'm not familiar with these engines.