Tony,
Its sounds like the otter switch....in the engine bay. Standing at the back of the car looking forward, down the left side of the engine is an alloy water pipe, ( it leads up to the large hose that goes to the water pump).....anyway.....if you look in the middle of the alloy pipe (preferably from under neath) you will see a little round switch inserted into the pipe...the otter switch..this should have 2 wires leading to it. Its function is to pass a current at about 90 degrees C, to turn the fans on. It may have stuck shut. To test it, switch your ignition on ( air con off) and see if your fans run from cold..they shouldn't.....if they do, pull off one wire from the otter switch and they should go off. If not it must be a stuck relay further down the line. New otter switches are available, Dave tells me that the German club have some that switch on about 75 not 90 C , which sounds a better idea to me.
Chris P