DJ6
I think ACL times are on and off stand times. There will be taxi time to or from the runway. I would guess from the slide LTNman showed that STN has a 10 min taxi time. So 23:45 departure plus 10 taxi out is 23:55 on the runway and 06:15 arrival would be 06:05 on the runway plus 10 taxi in.
A 6 hour closure seems a pretty short closure so I would think the airport would expect late runners to divert. They can't risk not finishing the works on time either on the night or on the program overall.