I have two good bits of news for you. One is that because the fan is still running, it means the fan and its power supply are in good condition. The other is that you have a mk1 8v, which means the fan and thermostat system are independent of the ECU or any other over-complicated wizardry. My guess is that your thermostat is faulty, and holding the circuit closed. First of all though, just check that all the wiring at the back of the radiator is intact and not causing a short. Brand new themostats are not terribly expensive brand new, and they just bolt out/bolt in to the radiator assembly at the back, between the reservoir and the fan coweling. With a deep enough socket, you may even be able to replace the thermostat without even removing the radiator (I don't know for sure since my car is in for its MOT, otherwise I would take a look).