At what stage is the project and how to submit a represenation to the Planning Insectorate.

The application is currently at acceptance stage. The opportunity for persons to register to become an 'interested party' will not arise until the application has been accepted by the Planning Inspectorate. If the application is accepted to be examined, the developer will be required to advertise in local and national press the period within which anybody will be able to make a 'relevant representation' and register to become an interested party. Notification of this period will also appear on the Planning Inspectorate's project webpage, here attachment 1
The Planning Inspectorate has published a series of advice notes which explain the process, including information on how to get involved. These are available at the following address: attachment 2. I would draw your attention in particular to advice notes 8.1 through 8.5.
attachment 2