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On the Mersey Roads 24 there was a small marquee on Prees Heath RAB for the unsupported riders. Basically you could dump a large bag/box in there and then have access to it whenever you passed it (all loops of the 24 passed it except for the finish loop). Some people had made up 20+ bottles so they could just nip in, switch bottles and straight back out onto the bike.
I used it (and slept it in for 90 minutes whilst tummy troubles subsided) when I did the 24.
Might be worth asking if the same thing is happening on the Nat 12.
I've found one Strava activity for the D12/1 and it looks like there's a middle section which doesn't go near the Prees Heath RAB. Looks a bit undoable on an unsupported basis if that's still the course.
The northern circuit is worse than Newbury, by a considerable margin. Lane-wide bone-jarring trenches all the way across the road. Plus potholes. The smoothest line is to try and ride along the rumble strip by the side of the carriageway. It's smoother than the road surface.