Installed 10.9.5 which worked without issue natively, installed the Logitech support / driver software for OS X from their website then installed Yosemite on top of this all as an upgrade. (Had to use strictly the keyboard in installer mode) After installing, booted up Yosemite and voila! full mouse functionality clicking and all.... f***** weird. Anyways. Solved, feel free to close this.