Yeah sorry, I made a mistake in my original post the hall file is:-
net hy-Temp <= spindle-vfd.Tmp => pyvcp.temp

So the XML file is right, but thanks it did make me check it.

I also added this to the Hal file:-
net hy-ACV <= spindle-vfd.ACV => pyvcp.AC-V

And this to the XML:-
<label>
<text>"AC - Volts "</text>
</label>
<tablesticky sticky="e" />
<number>
<halpin>"AC-V"</halpin>
<format>"4.0f"</format>
</number>
<tablerow/>

I get ACV data when the VFD runs (albeit in the thousands instead of the hundreds eg. 2100 ACV instead of 210.0 ACV), but that is something I can fix(I think).