As you have been advised on your other thread, I would refer this to the council. It is another example of poor management that you can add to your list.
They may only be the landlords but they do still own the allotments and have ultimate say over what goes on. If you couch your enquiry in terms of ‘As a responsible plot holder I need to know what our insurance covers so I can ensure I am not risking ….’
Try and keep it civil and calm and while posts encouraging confrontation and even physical responses might be amusing, it can just as easily backfire. You can end up in the wrong the minute you lose control and start shouting and come across as no better than the bullies.
I know it is a difficult thing to hear, but there are 2 sides to every story. This pair of bullies may well be being egged on and encouraged by others (the chair for one) or they might not realise the impact they are having and genuinely believe they are doing the right thing. Is there any way to get together as a group and speak to them somewhere neutral and see if they are as bad as you think. If not, I suggest everyone that is unhappy complains to the council so they get a mass of emails in one go. That should spark something, even if it just a warning to those involved.
