Does beg the question what he was doing to get caught with it in the first place? Can't imagine the Surrey old bill would have random sniffer dogs at a kiddies fun park on a random Monday.
When sentencing, justices must give a breakdown of what's given for each offence. I imagine that the suspended sentence was for affray (being a more serious offence) and it's quite possible that there was "no separate penalty" for the possession.
If that's the case then to activate the sentence the new offence has to be of a similar type (although the caveat "in the interest of justice" applies).