There's been a lot of discussion here about leaking problems - probably not just with the Ego-C, but with the tank design generally?
I sometimes also find, at the join between the tank and the cone that it's wet! After some research online, I came across these two possible solutions provided by two American vapers. I haven't tried them myself yet, but wanted to get this up here while I had a quiet moment:
The first deals with the end cap itself, on the tank:http://www.youtube.com/watch?v=o7bwpkM3dr8&feature=related
And this deals with how the tank is re-attached after filling (he describes it at about 4' 30"): http://www.youtube.com/watch?v=Sjd1R_-uuIE&feature=related
Whether both or either one solves the leaking will have to be tested - fingers crossed! If anyone can't view these videos, I'll post a summary. Finally, apologies to anyone who has already posted these (possible?) solutions, but I couldn't find them on here.