It does say 400 yards in the book, but if you push your plane back into the boulders you get the extra distance. Its like a displaced threshold, and as WR said it depends on your interpretation of "rough"! Also the wall at the east side is less than 4' high, so i expect they make overly generous allowances for that. Lastly, if it was a bit marginal, you can jink it up over the wall without worrying of a stall as you are straight out over a 300' cliff
As i said before, i would definitely be up for a meet on Lundy!
Who's coming
I have heard that B+B is available on the island, although i hven't tried it!