This is quite likely to be the fault of BT Wholesale not notifying Plusnet that your migration has completed (a not infrequent problem). There used to be a ticket path in the help assistant to notify Plusnet that a migration had completed - this doesn't seem to be there now.
Have you explicitly in your tickets in a simple and clear way (not buried in the middle of a ticket with other information) notified Plusnet the day your transfer completed?
Once Plusnet have got confirmation of the date of the migration I don't anticipate any problem with you getting a refund. I don't know if it is still the case, but they used to take in full any payments due during the 30 day notice period and then refund the difference afterwards when the migration date was known.
jelv
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