Pete, I have used that 12/300 in the most Iron infested sites in the EU and found 3 items of Gold in One Sweep Of The Coil In and Area that has been used as A Local Test Area For Every Metal Detector made in the Last 47 years and I found more Roman stuff in One week than I found in 6 years in the same field.
What I am saying is Just because its Big or Junk filled site Don't Right it Off, when the signal is the loudest the Item is right in the middle of the 12" Coil, and being a Concentric NOT a mono It wont be effected by EMI where a DD will,
It,s better than you think, Don't try to Hard and relax and trust the Machine, if the item is within 6 to 8 inches or big enough, IE coins etc it will tell you the truth.
Good Luck,, john
PS, The Book is Good but take most of it with a pinch of salt until you are at ease with the machine, Then you can Identify with what he is talking about, Ok