Was the date on the final bill dated before the cancellation date of the 27th March? Or even on the day?
The cancellation can take place any time in the actual day of disconnection but the bills are produced early hours of the morning do this is possible.
Final Bills are produced within 3 working days of the disconnection so you should receive one soon if the one you have had bills you for the advance period
If you've overpaid in advance any money owed to you is refunded via cheque once the account has been closed for 35 days. The cheque is then processed and sent yo you so is usually with you round day 45 after the final bill is produced.
More info on Final Bills can be found on that links to the VM help article about it.