Personally, I think picking 9am to start is only worth it if you are relaxed about getting 2.45pm instead.
Pondering this a bit more today, I think once I come allocate times there's merit in reallocating people from the least over-subscribed groups first, and moving them to their nearest free slot. At the end of the process you would have people from the 9am and 6am slots filling up all the remaining slots, likely to be 7:30am and 7:45am, and 2:30pm to 3:30pm. This approach might nudge people towards more caution and perhaps result in fewer surprises.
A couple of people have decided to leave the matter to fate. I think that's a grand idea. That means they'll start late and enjoy the controls at Spalding and Louth.