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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.

Don't forget that the resistor gets pretty hot, that's why it's located inside the unit. The flow of air keeps it cool

I need to sort one of these out for the GTM as it is suffering the same issue.

The way it is mounted, it is open on three sides, probably better air circulation than the original installation. I also found a unit for Ford Transits wound on a ceramic core which could probably handle twice the power

Mines exactly the same, though I was considering a different fix, anyone know what current these motors draw?

On my Gold the heater fan resistor is mounted on the shelf just besides the heater fan unit. It is not mounted inside. It even has a steel flap that probably is needed to get rid of the heat. I am sure it pulls a lot of amps. On my car if I switch the fan on, the window wipers will slow down. Apparently something is wrong with the wiring on my car as well because I did not have this problem on my 2 previous Golds.

I have checked voltage and current at heater settings today.
On full speed there is no current going through the resistors, so no heat generated there. At low speed, the current through the resistor is 3.0 amps and the voltage drop across it is 3.9. Therefore power dissipated approx 12 watts. At medium speed, the amps are 4.8 and volts 1.9, therefore just under 10 watts dissipated.

Cheers mate, thank you

At the moment I'm reconditioning the heater unit for my MK 2. I would like to modify it so at least I have a 2 speed fan. I'm considering purchasing one of those Rover resistors off eBay and an appropriate switch, I'm just wondering where the best placement for the resistor should be considering how hot they get. Anyone got any ideas.

I used one of those Rover resistor units and mounted it on the side of the battery box with the plastic mounting uppermost and open on the sides. When I went to the classic car show at the Bath and West showground, the demister was on all the way down from Birmingham on the middle or low speed with no problems. Remember, even on the lower settings, most of the power will still be dissipated in the heater motor and at full power, no current goes through the resistors.