Gazz as I said I'm pretty certain its a crack, if you wanted I could check it for you and put you in touch with welder who could repair it or at least tell you if it could be repaired. It looks like the weld was slightly undercut during fabrication.
Years ago I was shown a cut open Cannondale frame by a competitor - the standard of the welding when seen from inside was horrible, which was the reason we were told for Cannondale having those big smoothed over and filed joins.
Can't say for sure obviously but that does not look like a crack - more a manufacturing F-up which was smoothed and covered over with paint.