It would be a shame if it didn't happen in 2018. It is a good vehicle to get into longer distances and boost memberships and awareness of AUK.
The level of organisation required must be on a different level. There are umpteen 400-600 in the calendar and all but 2 are basically x-rated events, not very inviting for someone who has never ridden overnight. Those two are consistently oversubscribed. There is big demand!
I have a lot of friends who would like to try something along those lines... but LWL sold out in hours and there is nothing left.
Here's a thought: failing a National 400, what about a National 300? Significantly reduced workload and with an early start at 6 AM, there is basically no need for sleeping arrangements, as the last riders would roll in at 2 AM. Does it need to be a BRM? In a non PBP year it doesn't matter, does it? And for non AUK members it matters even less.
My feeling is that the calendar could do with more 300 rides. I would definitively do more if they were there. Ultimately 300, rather than 400 is the "step up" distance. Most people have done one or two sportives or audaxes of 150-200 km, but never gone any further.
BTW: I was hoping to volunteer at next year National 400 to give something back... assuming it's not going to be held in Scotland