I would assume that if it is currently working with a Virgin SIM it is either locked to them or already unlocked? If you call 789 from the mobile and can answer their security questions they should be able to help.
What's telling you that the handset is locked? If there's a Virgin SIM in the handset you can dial 789 to get through to the team free of charge, if not you can give the team a call on 0345 6000 789 to request that the handset is unlocked.
Tried twice now, 1st said no problem wait 10 days or so for code, that was 6 weeks ago, 2nd said IEMI I gave didn't exist and put me thro' to apple automaton , she said I quote " I'll rip that up " then hung up on me. Think I'll just call it a day now and avoid virgin In the future