Claim a tax refund on your pension by post.
Individuals who have paid too much tax on private pension lump sums or cash payments may be eligible for a refund. If you cannot claim online, you must complete and post the relevant HMRC form to your specific pension situation. Using the incorrect form may result in delays to your repayment.
Source: https://www.gov.uk/guidance/claim-a-tax-refund-on-your-pension-by-post. Summarised automatically from the official source — general information, not advice.