I'm not sure that there is anything in print, it just has to be in the unit SMS and having the safety case approved by the head of your regional CAA SRG.
As far as I can see, it's comensurate with the no of and type of aircraft movements at you unit, along with the way in which unit standards are maintained, i.e. how many LCEs are at the unit, do you do your own quality checks etc. or do SRG come to do annual checks etc.
I think it really comes down to the integrity of the unit SMS.