Hi Digeroo,
It sounds like these people are doing a weekly shop on your site.
I put a fence around my fruit plot to stop this casual browsing after
we had a spate of it last year. Stealing of any kind can get a person
seven years in prison and therefore you can make a citizens arrest.
You can also dial 999 for a crime in progress. And otherwise inform plot holders and the local police.
You can also take pictures and video from a distance if you don`t feel able to challenge people, but you can also tell the police you feel intimidated by these people.
Keep us posted, Col