I haven't filed nothing in.
So Ill expect a cheque soon 
Thanks 
They do the calculations automatically, and then inform you of the rebate/underpayment. (when they're not on strike)
If you DIY, you complete form P50. Mainly if you are on PAYE and have not worked the full tax year.
You give your bank details on this and hence auto payment.
You can claim back for the previous 6 years.