If you accidentally pay too much into a cash ISA during the tax year, you won’t be eligible for any tax-relief on the excess amount. But it’s not anything to worry about. In the event that you do overpay, you can get in touch with HMRC by calling its helpline on 0300 200 3300 from 9am to 6pm Monday to Friday. Alternatively, HMRC will contact you at the end of the tax year and notify you of any tax you’re required to pay.