The engine earth runs from one of the starter motor bolts to the battery negative terminal and seems solid. I disconnected it, put the continuity tester on it and it is good, both ends seem ok and feeling the cable itself, it doesn't seem damaged. The battery earth to the inner wing is solid also.
Thanks for the help. Gonna see how much ford want for a new coil altho i suspect that my original and the 2 second hand ones i've tried can't all be faulty?