Gotcha

Went out with the dog for a stroll past the site tonight.
Found more stuff that should have been on people's plots outside the site. Went in to talk to some of the plotholders and ask them to report anything to Plod, two of them pointed out a group of kids trying very obviously to be inconspicuous and sneak along the footpath behind the bushes and said they had been in when they arrived and had been hanging around outside waiting for them to leave.
Recognised two of them, they have nice responsible decent parents who have had words with them and told me to ring them directly if they are seen again. Just need to track down some of the others now.