Thanks, Rich.
Another option is to stop short (of Chepstow) so arriving at a 'reasonable' time [eg] Bwlch (an info control at 310km)
I think 310km is too early to stop, you'll be out of time at any control before 450km unless you set off way before dawn.
I think the ideal distance is about 360km makes day 2 easier and gives reasonable amount of time to make subsequent controls.
I agree it'd be good to crack on to Chepstow (and Rich has designed it so that the main entry can sleep at Bulwark). But for those 'johnny come latelies' without that sleep stop option, Aust is that bit further and would probably be after midnight depending on how things go. I guess the backpackers hostel in Chepstow is an option but not sure how they are with a midnight arrival.
Sunrise is at 04:31 and first light will be an hour earlier than that, in benign weather. Chepstow at 368km at 15kph has an 'out of time' time of about 06:30. I'd plan to get 5 hours + sleep, be away by first light from Bwlch and have breakfast in Chepstow.
On the BCM last year I stopped short (at 340k at 10pm) slept and was on the road by 5 (sunrise) and at King's (374km) by 6:20. Worked for me.
With regard to 'Great Natural Wonders of Audax' I will treasure last year's experience of leaving the YH/optional control in Alston (after a great night's sleep) and hitting the top of Yad Moss dead on sunrise.