That would have been out of sequence then. The devices would have needed to enroll into the server with an Enrollment Rule that was associated with the Enterprise Binding so that they could get Managed Play service accounts assigned at enrollment. If you did add the Enterprise Binding until post enrollment then you obviously would not have been able to enroll the devices under that Binding. 

You can factory reset the devices and re-enroll with an updated Enrollment Rule that has an association with that Enterprise Binding. 

Alternatively, you might be able to retroactively add the account to a device with the Reset Account device action. This is a newer feature that I haven't had to use personally yet so I can't speak to its effectiveness.

When the devices were enrolled were they enrolled under Android Enterprise and associated with that binding, or did you add the binding after the fact? Do the devices have an associated MGP service account?

Enrolment was done in the following method:

• New Device out of the box
• Enter afw#mobicontrol for the username
• Follow though

After having enrolled the device is when I've done the "Android Enterprise Bindings" as I didn't know that was a feature I would have wanted / needed. 

Many thanks Matt :-) 

The reset account worked :-D