I recently bought a very similar top and had to go up one size. When I mentioned this to the sales assistant, she said that this was a common issue and they'd had lots of comments from women having to go up a size in tops, and not being very happy about it.
However I did buy some jeans from there a while ago and I found those true to size, so it could just be that they make tops on the small size (or it could just be me). Probably not much help, sorry!