and...... I got it.
Firstly ensure the toggle on the side of the VFD is set to VI (wont run otherwise)
Settings on the VFD are now:
PD000 - 0 (not 1 as mentioned elsewhere thinking it's an unlock, cos it's not)
PD001 - 1, for external control
PD002 - 1, for external control
PD003 - 400
PD004 - 400
PD005 - 400
PD072 - 400, max freq
PD073 - 120, so it doesn't run too slow
so my issue was having a min speed set to 120 and then trying to set more than that using the PMDX-407 which the VFD was ignoring cos I had a min set.
It's WORKING :-)