Found it eventually!
The vdmadmin command is located in the c:\program files\vmware\vmware view\server\bin directory of the broker server.
For syntax command see vdmadmin -help
To remove a User from a dedicated machine:
vdmadmin -L -d <desktop pool name> -m <virtual machine name> -r
would clear the User assigned to that VDI and leave it open for reuse. Handy if someone was to leave the company.
It may also be sometimes beneficial to assign a temporary user to the newly freed up VDI to stop another User accessing it as it is now available. The temporary user muct be an account already set up in AD, such as a dummy account. Such a case might be where there is an issue with the VDI in question and don't/can't remove it from the desktop pool:
vdmadmin -L -d <desktop pool name> -m <virtual machine name> -u <domain\user>
Hope this helps someone down the line!
Paul.
