We really enjoyed the Erlestoke 12 race in 2013 (and 2012 and 2011). It’s back for 2014 and at the same great venue; a wooded hill on land owned by the MOD near Salisbury plain.
There’s a choice of 12 hour and 6 hour races plus categories for solo, pairs, teams of 4 including male, female, mixed and singlespeed (makes sense). Erlestoke 12 website (not updated for 2014 yet).
More info on xcracer.com.