Hannigan was Director of GCHQ November 2014 – April 2017 and stepped down at the end of April 2017. There have been questions as to the true date of the letter in question.
https://en.wikipedia.org/wiki/Robert_Hannigan
Hannigan was Director of GCHQ November 2014 – April 2017 and stepped down at the end of April 2017. There have been questions as to the true date of the letter in question.
https://en.wikipedia.org/wiki/Robert_Hannigan