Hi Leamcon,
As you are not resident in the UK, the tax deducted by your pension provider from your lump sum can be repaid to you.
You will need to download and print of the DT individual form here:
Double Taxation: Treaty Relief (Form DT-Individual)
You should declare all your UK pensions, including UK state pension on the form and send the signed and dated form to the Maltese tax authorities. The Office for commisioner for Revenue, will note your sources of UK income and validate the form, to confirm you re resident in Malta and taxable on those pensions there.
They will return the validated form to you, so that you can send it to HMRC at the address on page 1 of the form.
When processed by HMRC, any tax deducted in the current tax year will be repaid by your pension provider and any overpaid tax from an earlier tax year, by cheque (only to UK banks).
Thank you.