not sure if its true or works but i had battery trouble today (drawing 10 amps plus with all switched off).
any way this guy said if you put a mk1 in 4th gear and depress the clutch, the car will start when battery is low ie just about cranking.
it could help by reducing cranking resistance, it may not work but try it next low battery time, nothing to lose if it isnt gonna start anyway, if anyone finds it works post a yay or nay.