Alan Gosling

Hi I completed my self assessment return and entered my foreign pension saying no tax paid on it . Then i was told i did not need to complete any additional pages . I also added in the main extra information notes that i had received it and it no tax paid. The summary said that i did not have any tax to pay as my declared UK pension and interests were below the limit. I also received a letter to confirm this. With my UK declaration and foreign pension I calculated that I owe about £2700 tax. It the past I thought I received a follow up letter from HMRC to rectify the foreign pension earnings. How do I rectify this so HMRC do not believe I have not paid tax required and then get fined. Thanks