There seem to be a few issues here, and none of them is totally straightforward!
In principle, if the council has made a decision to hold a meeting on a certain date, and has not made a subsequent decision (as a council) to change it, then the meeting should go ahead. This isn't as clear cut as it sounds, though. If most councillors don't turn up (perhaps because the chairman has told them it won't take place) then the lack of a quorum will prevent the meeting happening.
The chairman does have the power to call an extraordinary meeting - which is only extraordinary in the sense that it is not one of the regular meetings planned by the council, typically as you say at the annual meeting for the year ahead. So, if councillors take the chairman's word for a change, then it de facto happens. Provided, of course, that an agenda has been published giving three clear days notice of the meeting.
Alternatively, if councillors refused to be told by the chairman that a meeting was altered, and the clerk published an agenda at the proper time, then the meeting would go ahead as planned, possibly without the chairman. And if nobody turned up for the extraordinary meeting, then it wouldn't take place.
Standing orders are a good guide and should normally be followed. In cases such as this, they can't be relied on because the council always has the option to suspend any of the standing orders that aren't a statutory requirement. So they're not guaranteed to always apply.
While the clerk should work with the chairman in, for example, agenda setting so as to ensure the smooth running of meetings, the clerk is not directed by the chairman. Except to the extent that the council has decided to delegate power to the chairman to manage the clerk. Even then, the clerk should be responsible to the council as a whole, and not to any individual councillor, over most matters.
In short, I doubt if the law has been breached, although I wouldn't think it good practice to keep changing meeting dates. Many councils have a fixed formula, such as the third Monday in each month, and stick to it almost come what may. If individual councillors, including the chairman, cannot make a particular meeting, then it goes ahead without them. The council is not the property of the chairman.
If councillors are collectively prepared to insist on a less arbitrary way of working, then it will not be possible for the chairman to prevent it.