Did you see this on STW? a new route through Northumberland, around 100m one way
http://singletrackworld.com/forum/topic ... humberland

A little googling has brought up
http://www.cyclepad.org.uk/listing/The-Sandstone-Way
http://www.northumberlandnationalpark.o ... itiesLooks like

Rich Rothwell has set the marker for a 1 way trip (11hrs) , mentioned at the bottom of his blog post
http://www.richyroth.com/pain-cave-nice-cave/

I'm pretty sure there's another thread about this...

Planning to ride this New Year's Day if anyone wants in