Range Rover Sport heater runs cool after period

[russfenwick]

I would appreciate any help before I have to open my wallet at the local LR dealer. I have a 2006 RR Sport. The problem that I am experiencing is with the heater. When I start off on a journey and normal temp. is reached the heater seems to work fine and gets hot. After a while (maybe 20 mins) the temperature coming from the vents seems to drop despite turning the temperature nobs up. On maximum heat setting (red dot) it goes warm but not hot. I have tried various combinations of fan/no fan, auto and various direction buttons to no avail. It also seems (but may be my imagination) to start to cool after I have negotiated a few turns. Anyone got any ideas please to try and narrow down the reason to cut down LR diagnosis costs? Many thanks.