Further to Ali Barber's post.
The RAF Board of Inquiry may or may not have invited AIB to assist with the accident investigations. However, even if the AIB had been invited to assist, I doubt they would have been allowed access to the wreckage at Binbrook until they had checked in with the President of the BOI.
I doubt very much whether the ejection seat had been "changed". I fully expect the ejection seat to have been removed to discover why it had failed to work and the instruments removed to discover whether or not they had failed prior to the accident and thus contributed to it.