Known corrosion point on the early Roadsters are various connections & a small white resistor box usually cable tied to the top crosshead behind the rad, mine was internally non-conductive within the 'crimp' and not easy to see visually, but enough to stop the fan cutting in
Morgan cunningly pre-empted this very fault. The high speed fan relay puts full volts direct to fan when it cuts in at higher temperature, bypassing the resistor fed circuit providing low speed fan. Only difference with your fault Jon would be a higher reading on temperature gauge followed eventually by deafening roar
