IMHO I do not see this as a blanked ban on the individual being in the planning meeting, as whether they have an interest depends on what is being discussed, whether it will ever get to the development management committee and by considering the various tests in the COC. Planning meetings can cover a number of local issues which are nothing to do with the development management committee and many applications which will not reach it either.
I think that this comes under Appendix B - Registering Interests, section 8.2
Where a matter arises at a meeting which affects - a financial interest or wellbeing of a body included under Other Registerable Interests as set out in Table 2 - which is - - c) any body
a. exercising functions of a public nature...... of which you are a member or in a position of general control or management.
If so the individual needs to consider
9. Where a matter (referred to in paragraph 8 above) affects the financial interest or well-being
9.1. to a greater extent than it affects the financial interests of the majority of inhabitants of
the ward affected by the decision and;
9.2. a reasonable member of the public knowing all the facts would believe that it would
affect your view of the wider public interest
You may speak on the matter only if members of the public are also allowed to speak at the
meeting. Otherwise you must not take part in any discussion or vote on the matter and must not
remain in the room unless you have been granted a dispensation.
The individual has to consider if the interest he has affects the financial interest or wellbeing of that body.
So - as noted, its up to the individual to work though this and then leave the room or not depending on their analysis, which is up to them and does not have to be explained to anyone.
If there is a contentious application which may reach that D&M committee the individual may leave the room to ensure they are not tainted by any discussion prior to it turning up later. But it is still up them.