[Laird Scooby]

Have you still got the main under-floor pump in the fuel circuit Tom?

If so, there is a Non Return Valve in the output pipe, IIRC this doesn't open until the pressure is above ~30psi so as the in-tank pump only delivers ~4psi (but high flow to feed the main pump) then you won't get fuel past the NRV unless the main pump is running.

From what i remember of the wiring diagram, the output from the fuel pump relay gets fed to 2 fuses on older cars and just one on later (single pump) cars you should be fine to jump across from the in-tank pump feed to the main pump but you might want to increase the fuse value to cover - certainly for test purposes anyway.
If you're planning on using a single pump system but only need to prove the engine runs, that will be fine, if you plan on keeping the under-floor pump until either that fails or the in-tank pump fails and then converting it to single pump then you'll need to make sure everything is safe electrically rather than just a jumper wire.