I will repeat again.

The earth for the fuel pump WILL NOT go to the ecu. Never ever. It will go to chassis/battery ground. It may go to ground via the inertia switch ( I can't recall if the switch kills ground or the 12v supply ), but that's irrelevant here.

If someone has wired it to the ecu, it is incredibly stupid and no wonder it will cause problems.

It is crystal clear on the wiring diagram I have here ( LHD 3 door ) that the ground wire from pump is brown, and is to same earth point as fuel sender unit also earths to the body. Diagram says close to the tailgate lock.
There is no mention of an inertia switch on this one, but I know some lesser EFI Ford's did still use an inertia. Not sure on a Cosworth

12v from relay is black/red