Thanks for your comments all!
PaulS, apologies for the filesize! here's an 800px wide version...
Jonathan, the shot was tripod-mounted, but do I interpret your advice about good tripod technique with a view to bracketing as one where no vibration (and therefore minute differences between the frames) is allowed to happen? (i.e. - cable/remote release, mirror-lock-up employed.) Good sense, yes, thanks!