The is a very, very poor excuse for an historical study.
Not only is the underlying research sloppy and incomplete, but many of the arguments persued are, at best, unconvincingly speculative and, at worst, downright laughable. D.J. Grey is obviously a great and passionate fan of both Scotland and William Wallace, but he is no historian and his book is insumountable proof of that.
If you're looking for a decent history of Wallace, stick to Andrew Fisher's definative study or the numerous and near flawless writings of Professor G.W.S. Barrow.