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The hall opens at 1am as the 300km ride starts at 2am, might be worth asking the organiser if it's possible to arrive on one of the last trains of the evening (the 00:25 from Victoria gets to Ashford at 1:58) and then kip in a corner of the hall (bring sleeping bag/mat which can be left in the hall during the ride).
(Pluckley is a bit closer than Ashford although not all trains stop there.)
I've seen people do similar in previous years (it's a long time since I did that ride and the organiser has changed a few times in between).
I once had a 50 minute kip in the hall before the 300 as I'd ridden 300km to the start of it (from Putney, via Cambridge) to make it into a "Flattest Possible" DIY 600.
Anyone doing the Fairy Fairly Flat rides on 23 June? Just entered then seen trains would mean getting to Ashford at 8 and the ride starts at 8.30 the start at about an hour/45mins after the start date which is annoying.