I know zip about Minxes…Do you have a parts manual for the Minx to identify the Rootes part #s? I ask because I have seen the Alpine rear drum brake springs available through parts suppliers. You could determine if the Minx used the same parts as the Alpine rear drums.
For Alpines with manual adjusting brakes (series 1 to 4 and late series 5):
Return spring cylinder end was part # 5037351.
Return spring adjuster end was part # 5037352.