There is a bit of a halo: that's what I meant by "tidying up … the processing". Interestingly, although it's easily visible onscreen (certainly on a full resolution file), it doesn't appear at all in print, even under a loupe.
The shot is a blend of a 15-second exposure for the sky and a rather longer one for the masonry. I need to do a bit of work on the mask. The longer exposure didn't actually blow the sky but it made it an awful lot less interesting.
Jeremy