Hi, Dave & Jonny
I've had these Cir. from nowhere at times to deal with also, Reorienting, editing them out in Mach after posting.
As far as Tricks and things to try, Someone (Sorry I don't remember who) suggested that after you load your dxf and clean & optimize it, Post it to Mach then, Use the "Mach3Load" Button/feature in LazyCam to retrieve it back from Mach and use it to do all your modifying and then re-post.
I've had some success with doing this with problem drawings, Give it a try.
Thanks, Chip