Thanks for both replies! The decision on the kit lens was that I sometimes found myself frustrated by ALMOST having a wide enough view for some situations, but not quite. Plus, when I did use my Tamron it was fairly often only up to 100mm, so the new lens should save some lens changing.
I dearly love my 50. I reckon I'll try the 15-85 and 50 in low light with flash and see how we get on. As I have a crop sensor, my 50 is pretty zoomed in, but I love it for portraits.