Hello, I am a solicitor with over 15 years experience. I will try to help you with this.
You have been working the allotment since 2011 & now they say it was a mistake! I think you reaction is absolutely right. In general contract law the time for them to claim a mistake is long past. By not allowing you to work the allotment for this time they have accepted your right to do so. This is the position on general contract law but your allotment may have specific rules that alter these general principles, eg there may be a right to take back the allotment for any reason on a set period of notice.