I had an SDLT (Option) advised by CDC Corporate in Oct 2011. HMRC queried it with CDC Corporate and according to communications agreed in a letter dated 12th April 2012 acknowledged there was a land transaction filed. HMRC then queried the use of SDLT on 11th Sept 2015, which CDC Corporate responded to. I now have a letter dated 31st May 2018 saying I have until 28th July 2018 to settle the outstanding Stamp Duty. Does statute of limitations not apply at some point?