You can make a claim for tax relief for employment expenses by post using form P87: https://www.gov.uk/guidance/send-an-income-tax-relief-claim-for-job-expenses-by-post-or-phone.
Certain claims can also be made on line: the HMRC website provides interactive guidance at https://www.tax.service.gov.uk/claim-tax-relief-expenses/what-claiming-for.
If you file a self-assessment tax return, you will need to make the claim in your tax return and if you do not currently file a tax return and your expenses claim exceeds £2,500 HMRC will require you to file a return.
Note that HMRC process repayment claims without investigation but may check the claim in detail later. If their later investigation shows that the claim is not fully justified, you will be required to refund the tax relief claimed and may be charged interest.
It is possible to make a claim for the current tax year and for previous tax years. The claim must be made within 4 years of the end of the tax year in which the expense was incurred.
Tax Refund companies
Some tax refund companies offer a service of making expenses claims for all eligible years and charge a fee based on a percentage of the refund that is paid. HMRC have issued a warning about some firms: https://dontgetcaughtout.campaign.gov.uk/claiming-expenses/. They are aware of some firms making excessive claims which result in a demand for the refund to be repaid by the individual taxpayer.
