I am not sure if any of the threads have indicated this before now, but at the GTR Meet the Manager this morning at St. Pancras (including coffee and cereal bar / fruit - which was nice of them), I was advised that the joining up of the Welwyn Garden City to King's Cross and the Blackfriars to Sevenoaks services will not now happen in the December 2019 timetable and will not start until at least the May 2020 timetable and that this was subject to the completion of testing and training on the ATO system through the core.
I got the impression that there were even more outstanding issues to be resolved before extending the Cambridge to King's Cross slow trains to Maidstone and reaching the full 24 tph Thameslink Core service. Apart from indicating that this was unlikely to be in the December 2020 timetable, no date was offered...