I do not believe that a post 1977 Plus Eight was fitted with a fuel pump relay.
It sounds as if you have access to a wiring diagram so that should be your source of reference.
The supply to the pump will be from one of the circuits that is open when the ignition key is turned on.
An alternative reference may be a Haynes manual for a Rover SD1. My 1976 Plus Eight's wiring diagram closely follows a Rover 3500 P6 even down to the cable colours. You may find that the same principle of using an SD1's wiring features was followed on your 1980 car.
