Finally got round to sorting out why my heater fan only worked on max speed. The wire wound resistor was badly corroded, and since to remove it would have meant removing the fan assembly, I looked for an alternative. I found on ebay one for Rover 100, 200, 25 and 45, costing the princely sum of £2.99 including postage. It is now installed on two simple L brackets fixed to the side of the battery box. At some point I will have a look in a breakers for the correct connector to make a proper waterproof connection.
I imagine that all Metro based Midas's use the same wire wound.resistor.