My understanding of the legislation in summary is that you need a radio telephony licence to legally listen to such channels and if you have one (and as this witness was apparently a pilot presumably they did) you have automatically signed the Official Secrets Act which forbids the person divulging the contents except to the relevant authorities on demand.
Whatever the legal theories it is perhaps a shame that the person who did this failed to remember that the family of the deceased would have to read her description in the paper.