Normally the 6 month rule doesn't apply if new evidence comes to light. This would allow parish councils to revisit their decision within the 6 months. This rule is to ensure that the same issue is not discussed time and time again due to internal politics / conflicts within the parish council if there is nothing new that would material affect the decision that has been made.
But if you parish council has specific standing orders to the effect that you refer to, they are breaking their own rules and would therefore, i believe, invalidate the decision.