Hi Neil,
Now we know what your query is about I've moved it to the right forum. I've also taken the liberty of breaking the text up into manageable chunks as I found it very difficult to read in one large block - and didn't want others to skip over your post for that reason.
Being brutally honest, I'm really sorry to say that I don't think there's much you can do to change their minds. From what you've said, it's a privately run site and therefore not likely to be statutory or to have council involvement.
It also sounds as though there's been a complete breakdown of relationships between you and others on the site - whether justifiably so or otherwise. Reading between the lines, you set out with the best of intentions to regularise the position but in the 8-9 months during which you were chairman things moved too fast for, and without the agreement of, the majority of the other plotholders.
In the absence of long-standing arrangements / agreements which state formal procedures for terminating tenancies, the power to make such decisions will in practice usually rest with either the landowner or the committee.
I note you say that you introduced lease agreements and constitution but, even if they did set out procedures, I think you'd struggle to get them to agree that they'd accepted them - or they may argue that they haven't taken effect yet. For example, when our council tenancies were altered a few years ago we had a very long consultation period and then 15 months' notice before they came into effect.
I think it would be entirely in order for you to write formally to the committee chairman, asking for them to set out in writing the termination of your tenancy, the grounds and a copy of the minutes of the meeting at which the committee decision was taken (this isn't a legal thing but it might make them think a bit).
The only other thing which springs to mind is whether you have approached the NSALG for assistance?
As regards the Insurance, that's in reality a red herring - it's taken us several years to get addresses changed in my committee! - and from an independent standpoint, 3 letters in 6 months is not significant in the great scheme of things.
Likewise, you can't prove who committed the damage so, other than report the incidents to the police and asking whether a community bobby could speak to the committee to ascertain whether they know anything about it, I would suggest there is no point in keeping that particular issue at the forefront. It must have been awful to see the damage but you asked for sound advice.