When you started with plusnet, you probably restarted the 10 day "training period". If there were any significant line drops duirng this time, then the IP profile would be reduced. (Do you keep your routrer on 24/7?)
Plusnet will (if you ask them) reset the system and put you into another training period. If you keep your router undisturbed during that time, and the line is stable, you should see an increase to the speeds you had with Virgin.
Tell them about those speeds when you ask for the training period to be restarted. Ask through the ticket system rather than by email.