There are currently two planning applications I'm aware of related to these developments:
22/00916/FUL
I am told that the deadline for comments is the 8th of July! So we need to act fast to have our say. You need to register and confirm your email address, which you should be able to do in a few minutes. Beware the server can get really slow at times, so don't leave it to the last minute.
My limited experience of planning says you are best of sticking to technical matters relating to the development, and the impact on the surround area & residents, referencing the documents submitted, local plan and so on.
I recommend joining the East of Chelmsford Development page on Facebook, as many of their members have been involved with this a lot longer than I have. I'm just trying to bring it to a bigger audience. You can also join our Save Sandford Mill facebook group for this petition.
Personally my comments will be a mixture of technical points, and expressing a bit of passion about how I feel about the area and why its important to me.