The bit I notice with interest is the mention of 319s;

4.8 Specified equipment Where the proposed amendment contains changes
to specified equipment (rolling stock), please give full details,
including timescales, and the extent to which the vehicle and route
acceptance procedure in the Network Code (Part F) has been completed
4.49-4.50
There are no changes to specified equipment proposed in this
Supplemental Agreement. All additional or amended services will be
operated with Southern’s existing specified equipment, primarily Class
377 rolling stock, however there is a possibility that we may use
Class 319 stock occasionally on these services.