Cheers Rob, if you ever get it out again, feel free to send the macro, that would be very useful!

I ran a few tests today, but I'm still confused. I measured the bit before and after a cut, with and without tabs, it's definitely not the bit slipping. I had 19.06mm before and after. Out of the two cuts, the one without the tabs was better with only 1mm remaining, the tabed cut had 1.6mm remaining. I copied my whiteside compression bit, which I know works, changed the pass over and bit setup to match the 3.175mm bit and ran it again. Still no difference. I even tried making a pocket 11mm deep and it was perfect.
I am at a loss as to what is causing it!
I will try and post the G code tomorrow to see if someone can see any issues with it, but as far as I can tell its correct. I might try just for sanity sake to run the g code for the 3.175mm bit with my whiteside bit in a piece of scrap to see if that cuts through the material to the 15.6mm depth, which might rule our the code.
The only way I can get it to cut all the way through is to set the Z height at 1.6mm below the actual z height of the material, so I know it can cut through.