Thanks for all of the responses. Just to be clear, I carried out no post-processing on the file other than a little gamma correction. The crop I provided is from the JPEG, but the RAW file shows the same effect, and most importantly, in-camera (I can zoom into the RAW image on the lcd screen and clearly see the dark edge). As for sharpening, I only applied standard sharpening in-camera, set to 2.
Looking at other photos I've taken with this lens, it does appear to be quite prone to green CA in certain conditions.
The weather has been terrible here today (strong wind, rain and sleet!) so I haven't had a chance to experiment further, but will hopefully manage to do so tomorrow. I'm keen to take some pics with different in-camera sharpening settings to see if that affects it.
Out of interest, is there a preferred in-camera sharpening setting?
@Parallax - that article on sharpening halos was fascinating. Thanks for the link!