Redundancy payments should be entered on supplementary form SA101.
Go to the HMRC web address: http://search2.hmrc.gov.uk/kbroker/hmrc/forms/viewform.jsp?formId=3180
and refer to SA101 (Supplementary Pages) PDF, and the following SA101 Notes Additional Information Notes PDF.
On page 2 of the SA101 form, under "Share schemes and employment lump sums, compensation and deductions", you enter the amounts as instructed on page 15 of the SA101 Notes, for example:
1. Redundancy payment = £20,000. Box 9 = £20,000; Box 5 = blank; Box 6 = blank.
2. Redundancy payment = £40,000. Box 9 = £30,000; Box 5 = £10,000; Box 6 = £2,000 (assuming basic rate tax applied to excess).