My car has the set up Craig has pictured, on my car above the tank is the plastic storage insert/tray, the roll over valve (18 on the diagram) didn’t have enough room to be mounted vertically, Morgan just left it flapping around in the hope it would jiggle around enough to allow the pressure in the tank to equalise, it didn’t , the tank would constantly vacuum.
I extended the fuel pipe and relocated the valve and it’s been fine ever since.
