Simon,
That could be correct.
If on warming up you had not quite got the temperature up to the bottom limit of the thermostat opening then the water would be flowing via the bypass into the bottom hose.
When the fan kicked in however obviously the water temperature should be at the point where the thermostat was open so the bypass should start to close and the main flow would be via the top hose back to the radiator.
Assuming your pump direction is correct (anti clockwise I think) then I think your head sensor is kicking the fan in before the water temperature reaches thermostat opening temperature.
Is the main hose from the thermostat to the top of the radiator getting hot or only warm?
Did you go for a good drive to get the water hot or just a drive round the block?
