I'm sorry, I wish I could help. I just had to say I am JEALOUS! No one should be allowed to look that great effortlessly AND have such a gorgeous bag to boot!!
Hard to say from that pic! It may be by Stella McCartney - one of the main fabrics in her S/S collection, was a cool, chain print ( printed with Stella's own, favourite, white and yellow gold chains).
Alternatively, Kate wears a lot of vintage items; so it could easily be something from the '70s.