When completing the return, please ensure that you are including the income and tax deducted as shown on your P60.
Also include any allowances there were included in your tax code.
You may have underpaid through PAYE.
Once you have submitted your return, allow 72 hours for processing.
You can then contact our Self Assessment helpline so we can view the calculation and advise of where the underpayment is from.
Self Assessment: general enquiries