Ah ha !
I have now got it (gonna need a name) talking to simplify3d !
Everything else I've ever used over a com port -real or usb - has talked at 115200
This thing talks at 250000 - figured that out, problem solved.

And it's apparently running marlin 1.0
I'm not impressed I have to say. Maybe it's down to the config file.

READ: Marlin1.0.0 READ: echo: Last Updated: May 10 2018 16:04:12 | Author: (John, default config)
Last Updated: May 10 2018 16:04:12 | Author: (John, default config)
SENT: M135 T0
READ: Compiled: May 10 2018
READ: echo: Free Memory: 12026 PlannerBufferBytes: 1232
Free Memory: 12026 PlannerBufferBytes: 1232
READ: echo:Hardcoded Default Settings Loaded
Hardcoded Default Settings Loaded
READ: echo:Steps per unit:
Steps per unit:
READ: echo: M92 X78.74 Y78.74 Z2560.00 E105.00
M92 X78.74 Y78.74 Z2560.00 E105.00
READ: echo:Maximum feedrates (mm/s):
Maximum feedrates (mm/s):
I've also moved the y stop forward 10mm.
So my final maximum build volumes are:
x: 195
y: 193
z: 180

I've also drilled and and lined the extruder shaft with ptfe tube.
Next job, heat and calibrate.