Hello Tom
The usual problem is the cable connecting the hearter water flow tap has a clip that jumps off so although you change the controle from hot rto cold the valve does not open
It is usualy this time of year we notice this as in the summer we tend to have the heater turned to the cold setting
I am affraid it is an up and under the dashboard job to conferm this fault
It is possible you have an air lock in the heater system but this is rare unless you have drained the cooling system recently and failed to put the heater controle onto the cold position first
My money is on the heater valve not being opened
Hope this helps
Roger L in Essex
Roger L
November 2006