There seems to be an issue with the different properties of maize and wheat and the stills had to be modified so that the wheat would not stick to the plates and gum up the process. It seems that the general rule with Scottich Distillers is to run with one type of grain at a time and not to mix them.
Ok, there seems to be a bit more information available and from page 86 is the following paragraph
"Barley-Although it can relatively inexpensive compared to maize and wheat, unmalted barley has been rarely used in grain distilleries because of the processing problems associated with high levels of gums such as beta(?)-glucans (Walker, 1986).
In grain distilleries barley is generally used in the form of malt, and its primary function is a source of enzymes to convert cereal starch from unmalted cereals such as wheat or maize into fermentable sugars. According to the legal definition of Scotch whisky, all of the enzymes must come from the malt, and no other externally added enzymes are permitted.
..........Since barley malt is a relatively expensive component of the production cost in grain distilleries there is a continuing drive to reduce costs by limiting the amount of malt used, and this has resulted in malt inclusion rates falling to less than 10 per cent in some cases. The main effect of this is that it is now more important than ever for grain distilling malt to meet high enzyme specifications"
So there you go, the mash bill, according to this book, is 90% wheat, red or winter, not durum and 10% barley malt. Sometimes maize is used but not generally. If you have any other questions please let me know and I'll go thru the book and try to find some answers and now I'm going to have a scotch.