If you have an offshore bank account and have not declared the interest (but should have) you need to Notify HM Revenue & Customs of your intention to disclose liability by 22 June, This should fix any penalty at 10% of outstanding tax (there will also be interest) If you miss 22 June the penalty could be 100%. HMRC can go back 20 years!
The letter from the bank has come about becauese HMRC now has access to account details for may hundreds of thousands of offshore accounts.
You may need to seek professional advice. Look at the Revenue website
https://disclosures.hmrc.gov.uk/oaics/ Barry Hallam