Thanks for the replies guys!
Hmmm, displacing is a great idea but I cant seem to think of anything that would do the trick other than maybe marbles which as you say, could potentially damage the flask, plus, unless they were tiny I don't think they wopuld fit through the neck.
@singlespeed That was my thinking and I would increase the vinergar/NaOH to suit.
Does anyone know the effect of dilution? My first thoughts is that it would increase the surface area/contact of the basal mixture with the naptha (I don't intend to increase naptha volume as more pre-evap would be required?) and therefore help pull the base into the naptha better?