Liba2569

I have called hmrc and they said I have an underpayment of £56.60 for the last year. But they will deduct £280 throughout the year because 20% of that is £56.60. When I asked the man on phone to explain he clearly had no idea himself as he just kept repeating the same thing. I want to pay the underpayment now so it doesnt affect my tax code next year. I don’t understand how underpayment of £56 leads to a deduction of £280. The man gave me a number to call but they don’t take payments through it and I cant find an answer to this anywhere. Please give me a number to call and make this payment

Hello, I have a full time job and in December 2023 I began casually freelancing, I earned £500 between Dec-Jan. I have not yet registered as a sole trader because I was under an impression I have until October of 2024 to do. In addition, I did not disclose my income as it was below the £1,000 personal allowance that I was told you have in addition to your full time job. However I have just been notified that I have a deduction of over £200 for the next year from an underpayment this tax year. Income from my full time job did not increase during the year therefore I am wondering is it due to the side hussle and if so was my understanding wrong and I should have disclosed this income? If so is it too late to contact hmrc and disclose it now? If the money was taxed it would be taxed on the gross I was paid I assume and minus the deductions for software/equipment etc I would of actually been at a £100 profit instead. Please advise, thank you in advance