It's better for distribution within the fuel system to add the stabilizer before filling the tanks. The M3W fuel system is a return type that constantly circulates fuel from the right tank to the injectors and back to the right tank. There is no inherent circulation of fuel from one tank to the other, but there is transfer of fuel between the tanks as the fuel level rises or falls. This transfer is accomplished by the alloy transfer tube down low near the front of the swingarm. So pouring the stabilizer into a parked up vehicle will not give a good distribution between the tanks as the fuel filler goes only into the right tank. You could take the machine out for a spirited end of season drive before parking it up and that should do a fair job of mixing the stabilizer with the fuel in both tanks.
It would be helpful here if Morgan had followed Ferrari's (mid engine) practice of suppling fuel to the engine from one tank and having the return line plumbed into the other tank as that ensures fuel (and any additives) circulate throughout the fuel system.
Also, practice of topping up the tanks before parking for the winter doesn't really accomplish anything with non rusting alloy tanks. And most of the fuel stabilizers sold claim to greatly reduce or eliminate evaporative fuel residue and fuel tank corrosion.
