It's hard to say with whistles really - it depends on the style of the dress. I'm a 12 but 2 dresses I have from whistles are a 10 and there is still room for comfort. Tried on another style of dress in a 12 and that was a bit tight - would've needed a 14. I find the tops relatively true to size though.
Have bought a jumper dress from phase eight in a 12, so will see how it fits. The "relaxed" look will be ok, as want it casual and will be wearing it after the birth of DC3 (more like a long, slouchy jumper - that's the plan anyway!), so I will be bigger than normal anyway, I'd suspect!!