For completeness, This has been resolved.
The issue on my side was ZTP.
From talking with our polycom sales engineer:
ZTP DOES NOT work with prov.userControl
-- this IS stated in the admin guide
ZTP is enabled by Default on every VVX phone leaving the factory
-- this was news to me.
I was under the impression that since we had not explicitly enabled ZTP that it would be off
So we have gone to Home - Settings - Advanced - Network- Provisioning -- (last line) and turned off ZTP and BOOM! prov.usercontrol works as expected.
to do this via your config file, you would add:
device.set="1" device.prov.ztpEnabled.set="1" device.prov.ztpEnabled="0"
as well as
prov.userControl.enabled="1" prov.userControl.postponeTime="01:00"
Had to post this, as I hate when i google an issue and find someone with the exact same problem who 'fixed it' but doesnt post their solution