A very interesting frame, hope we hear and see more about it! I have a 1970 Witcomb of London that had a small crack in the seat tube similar to yours, apparently from having too large a seatpost forced in. Hasn't been a problem for me now that I have a proper post in it--27.0 and not 27.2 . I did have a tiny hole drilled at the end of the crack by a local frame guy, but not sure that was even necessary. Given the seat stay placement on yours would you be able to reach the end of the crack with a drill? In any case I hope you get your sorted out, or get comfortable with it, as that looks like a very interesting and high quality frame.