(GMO's / Genetically engineered foods or ingredients)
This is what I have found out by calling / writing to various food manufacturers. Not an exhaustive list, and only correct at the time if they didn't lie to me.
La Fe Corn Tortillas (the "no preservatives" one) - GMO free!
Called 6th March 2014
They insist no GMO's, and claim to be certified. Hopefully they'll put it on their packaging.