My uncle died 12 years ago, at the time the executor paid inheritance tax but the estate passed to my uncle's widow. The HMRC sent the tax back to his widow. The executor did not realise this had happened, so when she died three years ago, the executor paid the money my uncle left to the beneficiaries without paying the tax (as he already thought it had been paid).
looking at the letter from HMRC it says I am liable under section 200 (1) (c and d) of the inheritance tax act 1984. How does this sit with the info you gave?