XK120 the official connection points on the earthing are as follows
battery to body in battery box area
body to bellhousing block bolt at rear of motor with braided earth strap and flag terminals
engine block to chassis with braided earth strap and flag terminals at front LH engine mount
The weakest link is relying on the body to connect power from battery box area to fire wall for starter motor
The simplest solution is to run another earth strap under battery box area to the chassis which will provide best earth connection
All painted areas need to be well cleaned
I have seen a car that the only connection on the earth was the oil and temp gauge capillary tubes - they got hot!
terry