Found the relevant extracts:
Time limit for charging person formerly subject to service law
This section applies where a person ceases to be subject to service law.
The person may not, after the end of six months beginning with the date he ceased to be subject to service law, be charged with a service offence committed while he was so subject.
But there is a magic bullet exception:
Sections 55 to 60: exceptions and interpretation
(2) Where any of sections 55 to 58 prohibits the charging of a person with an offence, the person may be charged with the offence if the Attorney General consents.
The AFA06 (enacted in 2009?) brought in sweeping powers. As I understand it if you have served, however briefly and however long ago, the Service can now come after you for any offence.
Incidentally being subject to Service Law is part of the X-factor calculation - perhaps we should now pay it to those who have left….