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?
You can also scroll down here to their wildflower seeds.
Here you can see a list of some of the many species of Oenothera.
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.