Hi Glenn,
Have you tried swapping the boxes around to see if the box will complete the migration to 360 in the location where the other box successfully completed the process?
You could also try the following which has worked for some forum members.
Turn off the TV set top box at the switch; then hold down both the power button and the + button at the same time.
Then turn on the box again whilst keeping hold of those buttons for a further 30 seconds.
The box should reboot, the power button will turn blue, and it should reset and boot up normally as a 360.
If that fails then you should contact faults 150 from a Virgin landline or 0345 454 1111, option 2 (Faults)
When calling Faults ignore all the options and don't press any buttons then you should be placed into a queue and an agent should pick up the phone and book the tech for you.
Or wait a day or two and a member of the Forum Team should pick this up for you