Next Tailoring Women's Midi Dress UK 6 Navy Dark Pinstripe Asymmetric.
I'd say it's a large 6 as I normally wear size 8-10.

Machine washable. 
Main: 63% polyester, 34%viscose, 3% elastane
Lining: 90% polyester, 10% elastane

Please note this is brand new and unworn and has been kept in plastic. While listing, I noticed the label has been cut and repaired (see final photos). This is how it arrived to me, brand new, from Next. 
 
Collection also welcome from SK13.