The writers of the South African guides have probably been the most split-willing authors anywhere, part of it relating to various species concepts. As far as I know, the last major taxonomic review of the Kelp Gull was in 2002 by Jiguet. He argued for the recognition of 5 subspecies (as followed by Clements), mainly based on biometrics (in particular the placement/number of the tongues on the inner primaries). Why this would lead to the taxon veluta being another species is a mystery to me. I'm not saying the evidence doesn't exist, just that I - despite actively searching - have been unable to find it. The only two references I keep on getting are:
1) The Sasol fieldguide. Being a fieldguide it presents no good argumentation for the approach nor does it lead to other references that might explain it.
2) "Recent work". So, there's probably some work going on, but it apparently hasn't been published yet. Admittedly, if they already knew veluta should be recognized as a species three years ago, I do wonder why it hasn't been published yet. Of course, if I somehow have missed the publication, I'd be pleased if anyone can lead me to it. I also notice that Clements doesn't accept veluta as a species either and he seem to be the most "split-willing" author of a World Checklist. Anyway, unless Steve already has done so, I'll contact the library mentioned in the previous post and see if an argument for the split is comming or it's just another split of doubtful origin, but to the joy of many birders wishing for another tick (and yes, I'd apply for an armchair tick if it really is true - but I wouldn't go there yet). Regardless, another photo of an individual from this group is only good, especially owing to the taxonomy.