If you disagree with HMRC’s review conclusion, you may be able to appeal to the First-tier Tribunal, usually within 30 days of the date on the conclusion letter. Check the letter carefully, as it should explain your appeal rights and the deadline; you will normally need to set out the decision being challenged and your reasons, with supporting evidence. If the deadline has passed, the tribunal may sometimes accept a late appeal, but this is not guaranteed. See the relevant guidance on GOV.UK or consider advice from a tax adviser or solicitor experienced in tax disputes.