[Philly]

LittleNicky, did you reclocate your battery? if not, are you sure there isn't a powerwire that may be grounded instead of connected to the battery?

If your M44, then your OBDII, was your m50 engine OBDII or OBDI? that could be something to look into.
EDIT : I see your in the UK, so this statement may be off all together... did m44's come OBDI?

Your extra earth ground might be a good idea but highly unlikely to cure your issue...
I have 2 negatives running off my battery terminal, one runs to stock location inside the battery box, the other one runs down to the frame rail - along with the one that goes frame rail -> engine mount arm. Either way would work.