Hi oniongrower welcome to a4a. I am sorry to hear about all your problems. You need to check your tenancy agreement to find out what your rights are but I very much doubt that you would be able to make a claim from the Council, as it is very difficult to secure an allotment site completely. It is wise for them to have some form of security, but nothing is foolproof with it having to be open to a large extent to members of the public.
You should always report any vandalism or theft to your council allotment officer or your allotment committee so they can see whether improvements can be made to the security, and also get in touch with the community police officer to report any incident. In our area Northumbria Police are very helpful and will make their presence felt on the site on occasion. Is there anything also that you can do yourself? I know how frustrating and heartbreaking it can be,having gone through it myself a few times, but it is a very common problem unfortunately, and everyone on here will sympathise with you. busy_lizzie