SWBKCB
I'll just recap on what I have heard in the past three weeks.
Originally it sounded to me as if the planned 'night freight' operation was going to be connected with Brexit contingency planning and would be relatively short term; that it would run from the middle of October for a month or two. It then became apparent that a lot of work was being put into converting the old Inflite hangar at SEN into an air freight handling facility capable of long term use. This news generated reports that it was for one of the international on-line consumer goods delivery companies. Amazon's name was mentioned together with the 'information' that two daily flights by B737-400F would be involved, with probably at least two daily movements being during the night period. Staff are now being recruited by Stobart to operate this cargo facility so something along those lines seems to be the case.
I've heard nothing more in the past week and perhaps this new thread will lead to more specific details emerging.