Sebabuilders

Hello. Last year, as a self-employed person, I started working with a company where I assemble kitchens and bathrooms for them. Including tax, I earned £41,295. They deducted £8,295 from me as an advance payment. They didn't include the approximately £3,500 I spent on materials. Despite this, I should have received £33,036 in my account. I did not receive this amount. I received £28,244 in my account. The missing £4,791 was transferred to my account on April 9, 2024 - i.e. in the next tax year. What do I do with it? I understand that I pay tax on the money I receive. If this is the case, I should pay tax on £4,791 this year even though it was already paid last year. Well, what about the material (£3,500) and tax was deducted for that. Please give me information/advice on how to solve this problem. Thank you very much Seb