Ford Ka 2006 1.3 OHC 67,000 miles

I only can get hot air from heater when engine turned off for a few minutes after it has reached operating temp.
Once I drive off heater runs cold again ??

I changed temp valve (ford part) last summer, and this problem has only just started, any clues ??

Rad fan cuts in when it should, and no loss of water or overheating.

Could it be the Water Pump??