Another possible prob is access at a certain time of the year due to closure of paths and part of the site for nightjar nesting protection. Go to the Sheffield Bird Study Group site's events/meets page for details of their annual 'Crepuscular Creep' at Wyming/Ash Cabin, which is an indicator of when some paths reopen (the Creep is fab, I always do it and saw seven nightjars the year before last)