How to claim back tax on a SIPP withdrawal for a non UK tax resident.

I just turned 55 and have just submitted a lump sum withdrawal from my UK SIPP and the funds deposited into my UK bank account. I am a resident in Singapore since 2012. I do not have an HMRC tax code, therefore an emergency tax code will be applied to my SIPP withdrawal. Am I right with the following: 1. My understanding is that Lump Sum pension withdrawals are covered by the Double Taxation Treaty between Singapore and the UK and so no UK tax should be due on this withdrawal. My SIPP provider has stated that they will deduct tax from the withdrawal and the level of tax deducted will depend on the tax code provided to them by HMRC. 2. Am I correct in saying that lump sum withdrawals from my SIPP are covered by the UK - Singapore Double Taxation Treaty? 3. What are the paperwork, documents or forms that I will have to complete in order to reclaim this tax back? 4. Do I need the forms to be stamped or completed by the Inland Revenue Authority of Singapore (IRAS)? 5. How long will the refund take to process and how will the refund be issued?

Hi,
Article 18 of the UK / Singapore tax treaty (Singapore: tax treaties) makes no mention of lump sums.  
This means a lump sum is taxable in the UK.
You can claim back any excess tax deducted by your pension provider by submitting a P53 claim.
You can do this at Claim a tax refund when you've taken a small pension lump sum (P53).  
When we have worked out any repayment due to you, we will send you a cheque.
We can send this direct to you at your home address or to your nominee’s address.
The cheque can only be paid into an account held in your name or your nominee’s.
Thank you.

I have tried to submit a P53 online using the Government Gateway but the ID verification was not successful. Should I complete the P53 form online, print it off and then post it to HMRC. What are documentations do I need to submit or additional forms that I need to complete in order to obtain a tax refund. Finally, how long will it take the HMRC to process the refund and send me the money into my UK nominated bank account.