Sorry to hear that you are having an unfortunate problem at your allotment.I have one as well, so you have my utmost sympathy.
I might be able to give you an easy solution to your problem.
Do you live or do you know anyone that lives near to the allotment ?
What I suggest you do is buy a WIRLESS camera system from Maplin at £69.99. (Two cameras supplied). The camera you can install in your shed and runs off 12Volts. All of this can be hidden. You take the receiver end of the kit supplied and plug it into a video recorder (remotely) and you can watch what is happening on the allotment ! If you have the time and date stamped on the video recordng, this can be used as evidence in court. Make sure you transfer the video recording on to a DVD !!
Any problems, feel free to contact me.
Regards, Graeme.