Thats the pressure relief valve, from memory a 10mm open ended spanner will do the trick. Has a tendency to leave its internals in the pump so a magnet is a must so you can reassemble it before reinstallation.

After what feels like years, I finally got Sandy up and running again. Turns out there is a bracket on the back of the fuel pump which wasn't installed. Got one to put on and boy did that suck. But it repositioned the FIP where she runs good...