As far I can work out, different insurance is required for different things:
Public Liability - what most allotments have - covers you for visitors to your site, not for allotment holders
Employers Liability - if you are doing work on behalf of your society, or are letting people use equipment such as strimmers, rotivators etc, they are then deemed to be "in your employment" and you need employers liability in case of an accident
Contractors - as in previous thread, they should have their own insurance, and this should be checked before they start any work
Hope that helps
Joolieeee