I agree with fiction-factory, it will come out in the wash. However at section 12 of the Self Assessment form there are 2 boxes, 12.8 and 12.9. These boxes ask if you want any underpayment from either the past and the current tax year collected via through wages or pension. If you filled in the paper return, the full explanation is on the back page of the Help document that accompanied the Self Assessment form.
An on-line explanation can be found here
https://www.gov.uk/understand-self-assessment-bill