While parish meetings are often conducted quite informally and discussion ranges freely, the legislation does support the actions of the chair and clerk. A NALC document isn't really needed to establish the rules, but may have provided helpful advice.
Where the parish meeting is called by six electors of the parish, the Local Government Act 1972 Schedule 12 section 15 (2) states that not less than six clear days (days excluding Sundays or public holidays) before the date of the meeting, public notice must be given, specifying the time and place of the meeting and the business to be transacted. It must be placed prominently in one or more public places, and signed by the electors calling the meeting. (The position is similar if the meeting was called differently, apart from the signatories).
From what you say, the additional items were not included in the public agenda, and so the chairman was entitled to exclude them. If they were important or contentious matters, it is reasonable that the correct notice should be required so that all electors have an opportunity to know what is about to be discussed, and to make arrangements to attend if they wish. If there are still outstanding matters, I'm afraid you will need to call another meeting! The parish council has to pay for the meeting room.