ID help needed please.

http://s1102.photobucket.com/albums/g444/bigalinsk/?action=view&current=salad.jpg

This spring I found a number of these volunteer plants in a bed where I had sown some veg. They are about a foot tall. Have purple stems and a nice crunchy texture to the leaves. The taste is mildly peppery, and
they are great in salads.

I can only think that they may have come from a pack of mixed salad leaf seeds that I had the previous year.

I would like to be able to buy seed for the future, so does anyone recognise these, please.

Al.
.