thanks for all your comments.
I wasn't paying particular attention to the temperature gauge on the run although it might have been showing a bit cool - round about the 80 degrees mark.
The heater knob seems to have just 2 positions with an 1/8th turn difference. It doesn't click (by feel or sound) like an electrical switch but it's a very small movement for a bowden cable. I'm thinking of choke & magneto controls on old motorbikes.
The dealer thinks it is the thermostat but gave the impression that it isn't a familiar problem.
So it's time to take another run to check the heater hoses then drop the dashboard to have a look at the heater control.
Check the engine top hose at the same time to rule out or confirm a thermostat problem.
If the top hose is hot but the heater hose is coolish then the problem is somewhere in the heater circuit but if both hoses are coolish I'd be looking at the engine cooling system for example the thermostat.
If the thermostat and engine cooling system are working properly the engine should get up over 90 deg relatively quickly even in cool weather and stay there.
