Regarding doubt on foreign income for UK employed and self-assessment

Hello, I am currently employed in the UK and my taxes are automatically deducted and paid by my employer. Recently, I have done an activity (is that self-employment?) in June 2023 in which I was invited to review few research proposals for a Poland based research projects funding-agency and did this activity online. The amount I was to be paid was 1,200 Euro - and after deducting taxes in Poland (source), I was paid 798.47 GBP to my UK bank account for my contributions in August 2023. As I have never filed self-assessment taxes before and am confused (never had to as have always had taxes deducted by employer), for this special activity, can you please clarify:- 1. If I need to file-self assessment tax return for this activity, and by when is the last date based on the dates I have provided above please? 2. Do I need to pay tax on this again in the UK - and if not, do I need any to ask for any documents from the Poland agency to show that I have already been taxed at source? They said that they can provide me a "Information on revenue (income) derived by a nonresident natural person" at the beginning of next year (2024). 3. What "article" number does this activity fall under the UK-Poland double taxation agreement for the self-assessment please? Thanks very much!

Hi Joyjit Chatterjee,
Please refer to guidance here:
Tax on foreign income for points 1 and 2.
Point 3, the article will be no4
Thank you,