$14.50 Inc GST
Part Number: 5L1137
All Minis require an earth strap from the engine to the body and from 1959-1996, they fitted from the gearbox to the subframe.
When they introduced the twin point in 1996 it was moved to the upper engine stabiliser arm. About 13"LONG.
If your battery seems to always be going flat and the alternator is okay it is probably corrosion on the cable ends which need cleaning or replacement.
If you have a twin point, add an extra cable as the hole in the front subframe is still there, and bolt to the first lower bolt on the gearbox at the timing cover end of the car.