Grand Voyager 3.3 LE petrol 1998.

now the sun has come out to play here in the UK the voyager has started to overheat - temperature needle reaching maximum and dashboard alarm sounding.

now there are a couple of peculiarities to this car - whoever had it before me has overridden the radiator fan operation and put a switch in the dashboard to operate it. in normal weather i have NEVER had to switch the fan on (normal being being 21C or lower outside). Since the overheating problem started (temperatures outside 25C) i am switching the fan on when i start the car but the car still overheats after 10 minutes driving.
the engine oil is normal colour, indicating no head head gasket problem
the flow pipe from the top of the radiator gets warm after running the engine (even when not overheating) indicating the pump is operating normally.
the car can sit on the drive and idle for long periods without a rise in temperature.
the car is handling normally, with no dra on the brakes and not towing. driving on the flat (it's all flat here!).
so everything indicates normal operation apart from the outside weather and the overheat - as a north american car though surely it isn't going to overheat in a mild british summer?!
any ideas as to what i may be looking for before i hand it back in to the garage again next week?!