Please see
http://towncouncillor.com/index.php/1473/can-a-pc-discuss-the-precept-allocation-in-private for general discussion on exclusion of the public.
In the case of planning applications, I cannot think of any circumstances where exclusion of the public would be appropriate. Planning applications are public documents (as are most related documents such as any submissions relating to an application). Planning applications must be decided on the basis of planning considerations, all of which are public. The parish councils views that are submitted to the planning authority will be public. So it is hard to see a justification for confidentiality here.
On a related topic, a planning application is clearly a matter where a councillor could have a financial interest, either directly or through family or associates. In that case, they would be expected to withdraw from the discussion and the decision.
