In the end to "fix" the problem I had to pull the printer apart. I'm gobsmacked at just how crappy the design of some parts of this thing are. Each color ink is made to pass through a pressure reservoir (the largest one is black, the other three are cyan, magenta and yellow).
When fully assembled, a spring loaded piston permanently presses against the flexible membrane of each reservoir. The design flaw, obviously, is the stupid lolly wrapper-like membrane glued in place over each reservoir that seals in the ink.
In order to get the printer going gain without dribbling yellow ink over every page, I peeled the leaking yellow membrane away, let the ink drain and cleaned the reservoir out (metho + tissues). I then filled it with 5 minute Araldite.