Interesting.
Not sure how to link the two, as I've not done a 'flat' marafun. That said, apparently you can take 45 minutes off Beachy Head times to get a flat mara time.
BB 3:52
Beachy 4:49 (= 4:04). Remove times for stopping for a pee, finding the biggest hot cross bun at the feed stations and avoiding the back ends of horses, and we're almost there.
My average pace was about 9 min/mi. Equals 3:55 mara.