Hi Spook,
Although it does look similar to evening primrose aka Oenothera, I'm wondering if you are located in the UK. I ask that because Suttons seems to be located there. They appear to have more then one wildflower mix. Do you know which one you planted?
http://www.smartseeds.co.uk/wildflow..._mix_seeds.asp
You can also scroll down here to their wildflower seeds.
http://www.gardencentre.co.uk/product.asp?id=2696
Here you can see a list of some of the many species of Oenothera.
http://en.wikipedia.org/wiki/Oenothera
Newt
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.