Most things are better sorted out by talking to people informally. I would not put anything in writing. It may do more harm than good and it will give him a reason to pick on you as that is how such people protect their petty power. Talk to committee members or the chairman and say that he has been a bit over the top and upsetting people. They may be fed up with him too.
well that's true but I assumed that talking to someone such as committee about it informally would already have been done, and fairly early on too if this chap is such a menace. And even telling him tactfully about his manner. In which case, those things have clearly not done the trick so a more formal approach might be considered.
But not taking formal action after other informal modes of action have failed just out of fear of inflaming things is fairly daft and pretty much encourages this chap's power games because he will know nothing will be done about it. Or maybe the chap genuinely doesnt realise what a pain he is being to folk and by not being challenged he remains unaware, so continues to do so.
Either way, you DO need to be prepared to list specific instances of his behaviour, with dates etc, in case the committee do want evidence of what he is doing if they are going to take complains seriously. And any committee worth their salt should take complaints seriously.
Although equally I know full well on allotments there is a perception of there being a very fine line between genuine grievances and petty issues.....and similarly some committee members who shy away from plot holder issues full stop if they possibly can with no intention of doing their "job" as it is often far too inconvenient for them!