well, Jazzy, not wanting to put the mockers on it, when the police finally decided to give the site a visit, they said it was because of the container (very large and stuckout a mile) being all padlocked up etc. that it just invited thieves! it was very much in the open. you cant win really. they gonna get in anyway, if they want to.

I'm not being funny/critical, but just some advice. something else we put in the rules was zero tolerance behaviour (think thats right) wedidn't take any abuse from anyone, if anyone didn't like what the committee did/says they had to put it in writing for the next meeting (we had one every month for the plot holders, so complaints/ect were seen to quickly.
the only way we found out to get things done was to keep pestering the council, its in their interest moneywise to get the plots,filled. we have waiting lists and plots are filled as soon as someone leaves., so they do not get overgrown.
I wish you well Jazzi you and your committee have to get tough, you wont be popular, but we found in the end a majority of people were behind us. and of course if peeps dont like and they can do better there's always the next AGM to put themselves forward.
good luck, and I hope you get that greenhouse sorted.
as a thought, will plotholders donate some of their glass? get a notice up and see!
P.

Sorry, haven't ploughed through all the comments, but would it be possible to set up hidden cameras to catch whoever it is?