Question

What can we do when an extraordinary meeting has taken place without public notification.

Answer 1

If proper notice has not been given for a meeting, then no decision can be lawfully made at that meeting.  Therefore any supposed decisions are invalid.

Direct enforcement through legal action would be difficult and costly.  However, it is unusual for a council to persist with unlawful decisions.  Councillors may be personally liable for costs incurred through actions that are not supported by a lawful decision.  Also, the council may rely on the questionable decisions in its future actions, and the status of these is rendered problematic.  So an unlawful decision is liable to haunt a council well into the future.

Your most likely source of practical help would be the legal officer of your district or unitary council that is the principal authority for your area.  They might well be in a position to persuade the council of the wisdom of taking decisions lawfully.

Answer 2

Probably nothing.  Parish Councils can't be held to account for breach of regulations by organisations such as the Local Govt Ombudsman.  Try contacting the Sec of State for Local Govt and asking them how!