Hi,
You will need to print off and complete the DT individual form at:
Double Taxation: Treaty Relief (Form DT-Individual once you are in receipt of your pension.
The completed form, should show details of your UK pension. You would then send the form to the Singapore tax authorities to validate.
They will return the form to you and you then send the validated form to HMRC, so that we can repyay any tax deducted from your pension.
Thank you.