UK Private Pension Resident in Spain

I moved to Spain on 04/07/21. This is my first UK tax year as non resident, I am now resident in Spain. I reported my non residency but have been instructed to complete a self assessment. My only income is from 2 UK private pensions. I have completed form SA100/SA101 form with the relevant income and tax paid as PAYE. I am struggling with the SA109. I have no foreign income to claim relief and the DTA shows I'm not entitled to tax relief. If I do not enter a figure in SA109 my return says I have tax to pay in the UK (using gosimpletax.com software). Can I still send this without paying? Or must I pay and await a refund?

Hi Steven Coster,
Please contact our self assessent helpline, either on 0300 200 3310 or via webchat at Self Assessment: general enquiries, to confirm if a tax return is necessary.  
You will be taxable in the UK on your pension from April 21 to 4 July 2021, when your pensions will become taxable in Spain.  
Unless you have other sources of income that make a tax return necessary, you may only need to submit a validated DT individual UK / Spain.  
The form can be found at Double Taxation: UK - Spain (Form Spain-Individual).  
The completed form should be sent to the Spanish tax authorities.  
They will return it to you and you should send it to HMRC, along with a "Residencis Fiscal En Espana Convenio".
Thank you.