payetaxpayer

Hi, For past almost 8 years, I have been a higher-rate (40%) tax payer (on PAYE) and auto-enrolled in my employers' pensions; and unfortunately was not aware of the need to explicitly claim the remaining 20% tax relief for my pension contributions. I have tried to call HMRC numerous times during working hours, but despite being on hold for quite long (30 minutes and longer), I have never been able to get through to an adviser. Therefore, I wanted to check: In case I am unable to lodge a claim with HMRC for the higher-rate pension tax relief, will I permanently lose out on the refund/rebate; or will I be issued a refund automatically at the end of the tax year (I'm ok to wait if it will happen later on automatically, but do not want to permanently lose it if that is not the case)? Thank you for the help.