Basically dismantled the unit to clean as hair gets everywhere, a small metal part fell out before i had the chance to...

So the obvious clue that i was missing - which i could not for the life of me find - was a pinhole in the metal bracket which slots over a corresponding plastic pin on the main body. The metal bracket sits vertically - not horizontally - and actually provides the second contact point for battery number two, to close and complete the circuit. I’ve added further photography below to try and illustrate how it is seated within the casing:

I’ve uploaded three short videos highlighting the inner space between switch and motor as well as the layout of the top cover in the hopes that this might reveal the proposed location of the metal bracket: