Found it! This is what I was referring to:
In the UK from CAP413:-
1.4.6 Following the initial distress or urgency message, it is permissible for pilots and controllers to use 'MAYDAY' and 'PAN' as a callsign prefix at their discretion, where it is judged that this would have a beneficial effect on the outcome.
Not quite as I remembered it, we are not in the UK, and as Herod points out it is common to only use the mayday prefix on transfer to a new controller, but my point remains. Wouldn't repeated use of the word "mayday" have helped ensure that there was no confusion as to whether an emergency had been declared?