Core Insights - Self-assessment taxpayers in the UK overpaid £8.9 billion ($12.18 billion) in income tax last year, primarily due to the Payment on Account system which requires advance payments based on the previous year's tax bill [1][2] - Approximately 2.6 million individuals paid too much tax through self-assessment, with many potentially unaware of their eligibility for refunds [2] - HMRC does not automatically correct overpayments, requiring taxpayers to identify and apply for refunds themselves [2][4] Overpayment Causes - Common errors leading to overpayment include incorrect salary figures and failure to claim legitimate business expenses, resulting in taxation on non-existent profits [3] - Taxpayers are advised to be meticulous when completing self-assessment forms and to seek professional advice if uncertain [3] Repayment Delays - HMRC backlogs can cause significant delays in the repayment of overpaid taxes, with some taxpayers waiting 18 to 24 months for refunds [4] - It is emphasized that taxpayers must actively check their returns and follow up on potential overpayments to avoid losing out on refunds [5]