Dalkeith is just over half way (thanks to the outbound dogleg). Lets say 720km.
At 12kph that's 60 hours. So it should shut 60 hours on from 2pm Sunday. That's 2am on Wednesday. Plus a few hours to account for staggered starts and anyone who has a legitimate problem that delayed them (hopefully not given the limited circumstances these extensions are handed out).
That means the bags from Dalkeith could, theoretically, be back in Cheshunt on Wednesday afternoon, assuming there's are driver(s) and van available at this time.
Thorne (at ~1100km) would closes 91h40m after Sunday 2pm. Thursday 9.40am. Give or take a few hours, and driver/van availability, the bags could be back in Cheshunt on Thursday afternoon.
I'm pretty sure it was only possible to add an extra bag drop at 3 of the middle controls. A bag drop at Gamlingay doesn't really make much sense given that it's so close to the finish. Anyone remember where you can add bag drops?