Main earth lead from gearbox to frame?

Hello folks,

I have been having issues on starting,it is very very intermittent meaning after 3 hrs driving it will just give problem,i changed ignition switch but did not help and no fault codes coming up at all,car is driving great and no other issues . i changed crankshaft sensor last yr,temp coolant switch,and starter..i might add i am by no means mechanical minded haha i done some investigating yesterday morning and found main earth lead was a bit corroded on the chassis under battery tray area which is connected to the gearbox from what i can gather research online,i cleaned it up and lastnight it just gave 1 start issue once after 4 hrs  and went fine for another 4 after it...i think it has made a significant improvement, as i said im only an not great with mechanics and locations but can the other end of the earth lead which apparently leads to the gearbox be accessed from engine bay or do i need to go under the car?i have obviously no ramp,i have looked online but cant see any videos or steps to get the job done...any advice please ? and of course do you think the earth lead is the culprit ? before cleaning yesterday after a few hrs i would have been cranking quite hard to get it started. The video is lastnight after cleaning, (the oil like flashing is sensor which gone since before last service,the guy was meant replace sensor in sump BUT FORGOT) lol