Hi ellieb23,
This depends on which accounting method you decide to use.
Most small businesses and sole traders use Cash Accounting, where the income is declared for the date it was earned and invoiced - in your case, you would file a 22/23 return to declare your Sept 2022- April 2023 income.
Cash basis
But you may decide to use traditional accounting instead.
Business records if you're self-employed
Thank you.