Gavin Williamson was sacked because he refused to resign; Mrs. May's reason for sacking him was :
'compelling evidence suggesting your responsibility for the unauthorised disclosure. No other credible version of events to explain this leak has been identified.' (my italics).
The investigation was conducted by Mark Sedwill, Cabinet Secretary and another close civil service ally of Mrs. May, and apparently at loggerheads with Williamson over the defence budget, so therefore not an independent investigation.
In his letter of reply Gavin Williamson writes:
'I am sorry that you feel recent links from the National Security Council originated in my department. I emphatically believe this was not the case. I strenuously deny that I was in any way involved in this leak and I am confident that a thorough and formal inquiry would have vindicated my position.'
If he is lying then he deserves to be punished, but this letter is carefully worded and has the ring of truth. Of Gavin Williamson and Theresa May, I would not, to my despair, trust Theresa May an inch.