Bit of an update. I think it's now sorted. There were a couple of issues, first was that it turned out the steps setting was 65kHz in Mach3, set that down to 25kHz. This became apparent trying a friends PC which was a little lower spec than mine.

The other issue was/is some binding on the leadscrew which I have improved by loosening the motor mount screws and running the axis up to the motor end and tightening the screws back up. I had to do this twice, the second time trying to do it on the tightest position, as the first time there was still some binding up at the top end.

Will see how it goes, maybe the binding will get worse again once I put the heavier 800W spindle on. But for now it's running the best it's ever run. I was able to speed the axis up to match the x axis.