Hi,
We cannot advise you on the complexities of Maltese tax. Article 18(1) advises that your UK SIPP is taxable only in Malta.
To claim the tax back from the UK, you will need to download the double taxation treaty form here:
Double Taxation: Treaty Relief (Form DT-Individual)
Please send the completed and signed form to the Maltese tax authorities for validation. They will return the validated form to you, so that you can send it to HMRC at the address on the front of the form.
Thank you.