EFI return line, use emissions return?

Dart,

Good point about not knowing which line drains the separator! If I can't come up with any obvious way to tell, that pretty much excludes some of the options considered!

It seems I have some sort of aftermarket cap, I'll need to check if it has any valve functionality or is just straight vented, there is no vent line on the filler tube as you describe.

The big issue about not having a vented fuel tank is.. collapse, which I have so far not seen any sign of! :) Having said that, there is the charcoal cannister return line, both open in the engine compartment and with holes rusted under the car... definitively need to check the ventability of mu fuel cap before I block the entry of this line to the separator!

My high pressure pump IS in the engine compartment, but it's being fed by the mechanical fuel-pump in the engine, something that seems to work quite nicely.