My cuts are not going to the correct depth. Checked the obvious things like a tight collet or any visible obstructions - but all clear. There is a horrible grinding noise every now and again on the Z axis that I cant seem to identify. The screen indicated it going to full depth, however it is only half way through the material.
Any suggestions?
I had an issue with X axis last week where it was not homing correctly and losing zero in between cuts. I replaced motor cable with the ONE spare in the box and it has been perfect since.
Which leads me to believe I may need to replace my Z motor cable - but I don’t have any more to test my theory. Any links to where these sort of cables can be purchased?
Did you check if the Z ball screw coupler slips? See 4) in this document. I would check this before I order cables.
Also you could check your cables with a multimeter (move cable ends while multimeter probes inserted, to see if resistance increases).
Make your own stepper motor cables
If you want to make your own, here are the specifications, but it is required to own the appropriate crimp pliers (see below). Judge yourself what is more expensive.
Soldering them will lead to problems especially because the machine lacks strain relief.
For correct wiring of the stepper motor cables on the Onefinity CNC, see wiring diagram here. Here is the list of cable connectors used on the Onefinity:
Anyway if you just buy the pliers and one die, usually it’s much less expensive than one original Molex or Amphenol or Phoenix crimp pliers, but you get still best industry quality with a Knipex.
Of course you can always get a no name crimp pliers for $25 everywhere, but in every case, please know that you won’t do yourself any good if you don’t
use a correctly matching crimping pliers / crimp die, that is made for the exact contact type you want to crimp, even when choosing a $25 model!
And please, also
make sure that the wire gauge you use matches the specification on the pliers/insert! Otherwise, especially at higher voltages, there may be serious electrical problems and hazards.
Note: Generally I don’t buy the crimp pliers from the contact manufacturers because usually they’re too expensive and from my beginnings with electronics on, I know the excellent ‘Made in Germany’ 
use a correctly matching crimping pliers / crimp die, that is made for the exact contact type you want to crimp, even when choosing a $25 model!