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I rode 300km to the start of the Fairies Flattest Possible 300 one time to make into a DIY 600 (in the days before ECE but which helped prompt the ECE system).
SW15 to Cambridge and back, then over to Bethersden via Tonbridge arriving an hour before the FP 300 started, quick kip on the floor and then out on the FP300. We were the last back by about an hour though, so I did feel a bit guilty about it, but this was still a couple of hours within the time limit of the calendar event. Would have got the train home but managed to blag a lift from someone.
I shouldn't be encouraging this kind of audax behaviour. I'm not sure I could ECE a 300 event into a 400. Sounds evil. Plus for a 400 there will always be a certain amount of night riding.
Anyway, these two (links below) start at night, you could get the train to an acceptable point and ride to Manningtree for the first one. Not sure how you'd get home without at least two trains. The second one you could easily ride there and back or do a longer loop to get there and train from Ashford once finished.
https://www.audax.uk/event-details/10002-green__yellow_fields
https://www.audax.uk/event-details/10384-fairies_flattest_possible_300