Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
6 sided sockets work better and also a whitworth socket(cant remember 3/16 or 1/4) is a "hammer fit" on a worn 12mm, used to use one all the time on rounded filter bolts
I have a set of sockets here that are for exactly that purpose..they kinda cam onto the rounded nut/bolt...never failed yet..even used them on stuck rounded sprocket bolts last night ...Might be a sealey product but can't remember