OK, thanks guys I've got it sorted!
I'll summarise for others, as I found it hard to find info on Active Low/High trackballs and wiring the resistors.
My Ultimarc trackball is active low, so after wiring the cables to the mouse PCB, I installed the 4 2.2K resistors (RED/RED/RED colour bands).
This can be done at the trackball end, or on the circuit board of the mouse. I thought the resitors had to go onto the ends of each individual X/Y cables and then connect to the PCB ie between the PCB and the trackball. This was wrong. They just have to take the +ve voltage to the X/Y lines.
I've provided a rather lame diagram for others that need help in the future.
Thanks again for the help guys! I'm sure you're sick of posting that photo Blakey!

