HMRC are reviewing each year separately and you may receive more than one tax calculation (on form P800). Two tax calculations will be sent in the same envelope. Unfortunately HMRC cannot co-envelope more than two tax calculations, so if you are due a refund for more than two years, you will get more than one envelope. However, they should be delivered at the same time.
HMRC will only send one cheque covering all the years for which you have overpaid and will add interest where appropriate. The cheque will be sent separately, after the tax calculations. If you receive a cheque, you should pay it into your bank or building society account.
Some people will already have had a bill for a later year - for example 2008-09, 2009-10, or 2010-11 or 2011-12. Wherever possible HMRC will take this into account (for example, setting off a 20110-112 underpayment of tax against an overpayment for an earlier year).
This will not be possible in all cases and HMRC are sorry that some a small number of customers will receive tax calculations that do not set off under and overpayments across tax years. If this happens, you will need to pay the cheque for the overpayment into your bank or building society account and HMRC will separately arrange for you to pay the later year's underpayment.