HMRC make mistakes. Once a mileage claim has been submitted, HMRC have the inaccurate view that you will visit the exact same sites and accrue the exact same mileage the following tax year. Based upon this assumption, they will change your standard tax code of 810L to a different code. In 9 out of 10 cases, if this incorrect code isn’t changed back, it will result in you paying an additional lump sum of tax at the end of the year. So don’t get caught out by HMRC’s mistakes – get in touch with Express Tax Claims to check your tax code free of charge.