We have been told by multiple people directly involved that 6 clear, full-colour photos were taken.
But I assume not all of them featured the RAF chase aircraft, just the mystery object in question.
Re the negatives, all you can say definitively is that their whereabouts are unknown (to you and your colleagues). Anything more is speculation which could undermine the credibility of the investigation.
Final comment about the MoD seeking to retain the redacted elements of their report until 2076. Whilst I understand there may be a desire to read into the existence of some nefarious scheme to hide 'the truth' from the public, there is often a more mundane, down-to-Earth explanation regarding national security. Don't forget the Govt didn't reveal the activities of Station X until the mid 90s, due in a large part to the groundbreaking work they did their which formed the basis for western intelligence gathering for the next 50 years.