Well, you're right, and I don't think the complaint route is the easiest, especially under the new regime recently introduced by the government.
The situation is that the people involved weren't really acting as councillors at all, since it wasn't a properly constituted council meeting.
I would tackle it from the district council end. Ask the district council how they have treated the "decision" of the parish council. If it is a planning matter, all submissions are public, so you should have no difficulty gaining access. You are entitled to insist that the disputed "decision" should be recorded as merely personal submissions unless parish council minutes can be produced to show that a council decision was made. This would also require the "decision" to be attributed to specific individuals, or dropped completely. If it is claimed that there is a council minute, then you can check that by using the elector's right to inspect parish council minutes.
