Flying Bull, cheers for that, but what's confusing me was that I learnt 7500 Hijack, 7600 comms failure and 7700 for a 'mayday'.
I just googled it
http://en.wikipedia.org/wiki/Transponder_(aviation)
7700: General Emergency[1]
7600: Lost Communications[1]
7500: Unlawful Interference (Hijack)[1]
yet the consenses on here seems to be as you say, 7600 hijack and 7500 comms failure
I learnt '75, taken alive; 76, box of tricks [ie radio playing up]; 77, going to heaven'