It's difficult to be definitive on this point. The powers of the chairman are almost entirely derived from common law and the standing orders, rather than from statute. So you will need to study the standing orders for your council, although they are probably very similar to others.
The meeting can only be closed when its business has been completed, although this could be achieved by agreeing to defer all remaining items to a future meeting. So, it would seem to me that your case is an adjournment.
The chairman does not have an arbitrary power of adjournment, but can only adjourn the meeting with the agreement of those present. If the chairman unilaterally attempts an adjournment and leaves the meeting, another member can be elected to preside and the meeting can continue.
But if the meeting broke up after the chairman's purported adjournemnt, there is little that can be done. In principle, it appears that arrangements should be made for the meeting to be resumed, although the remaining items could simply be incorporated into another meeting.
It seems that the key point is that if other members wished to continue the meeting, they could have done so.
