I'm sure it's actually quite easy to find. It's just a question of navigating the field boundaries correctly! Obviously, I don't have the McN-C book here at work, otherwise I could give more precise instructions. Maybe tonight when I get home...
It's probably best to study the OS map, alongside a higher res map like Magic or the CRoW maps which show the field boundaries properly.
I might have to try again myself in October now :-))
BTW, details of the Well are here: http://www.megalithic.co.uk/article.php?sid=8092
A.