MG Car Repair: Starter motor engaged while driving, speed hump, electrical fault

Question
I have a 72BGT. When driving her yesterday I heard a loud high pitched squealling scraping noise and experienced a loss of power and of electrical power. I later identified this noise as sounding like if you were to turn the ignition key while the engine was already running. This problem then stopped and I continued home. It then started again as I pulled onto my drive. I turned off the engine, left it for a minute or so and tried to start her again and all I got was a click from the starter motor and she will now not start. I am going to try and bump start her tomorrow. Is it possible that an electrical fault between the ignition and starter made the starter engage while driving or could it be the starter itself that has died? I have done some research and not found anyone else who has had this problem.

Answer
Hi Sam.  You are not the first person to experience this problem.  Your 72 BGT has a later type starter which throws out before starting to rotate, and does not disengage until you let go of the key.  It sounds as though yours did that by itself while you were driving.   Once the starter motor is engaged, it will rotate at high speed, driven by the car engine.  If this continues for any length of time, it will overheat and be ruined, so if it ever happens again, stop the car immediately.   
 
Unfortunately, if you fit another starter motor, it could well happen again and destroy the new starter motor.  You must trace the electrical fault as well.  

I would guess that the fault lies in the starter relay which lives near the fusebox.  If it operated itself (eg when you went over a speed hump), that would do it.  It has 4 wires going to it, including a White/Brown and a White/Red.  Wire the White/Brown terminal to a bulb instead of to the starter, turn the ignition on, then tap the relay with a small hammer.  If the bulb lights up, you have found the fault.