Yes but am I correct to assume that when logging the 1500 hours there is an instructor/qualified pilot to hand?
No, not at all. In fact, an instructor is present for very little of it, as a rule.
The training it self is very complicated but I am trying to paint with some broad strokes. This may or may not be correct.
It is incorrect.
Additionally, while you indicate that you're not interested in the classroom training that a student undergoes, you're missing a big part of the picture. The training that occurs is most certainly not all in the airplane. Instruction given in a flight training device, for example, isn't flight time, but today a great deal of training takes place without ever leaving the ground. Entire type ratings take place without ever having set foot in the actual airplane. Indeed, a pilot arriving on the line with your local airline may have never been inside the airplane he's flying; today may be his first flight. The rest has all been simulator time, even though he's 100% legally qualified to fly, with type rating in hand.
The ground training received is very crucial. Did the pilot simply get handed a book and told to go read, or was he given a thorough course of instruction? How did he do at that instruction?
I've seen a number of applicants wash out long before ever reaching the airplane; the ground training is a crucial part of that.
Any suggestions about the hours of ATP training over the years?
The actual training on the ATP is often just a few hours; just enough to pass the checkride, which is a glorified instrument checkride, which is something the applicant most likely did right after his or her private pilot certificate. Then again, the commercial is little more than a glorified private. Not a lot more training is involved.
It's the big picture that counts. There is no correlation between the total time and the amount of training received. I've worked with 15,000 hour pilots who couldn't manipulate the rudder pedals properly, and I've worked with very low private and commercial pilots who were whiz-bang sticks with excellent attention to detail, good hand-eye coordination, great instrument skills, and who were very competent aviators.
The military turns out aviators into very complex equipment with very low hours; the hours and the skillset do not necessarily correlate. The nature of the training is very important. One cannot compare two 250 hour pilots on the basis of their flight time, nor even on the basis of the hours of instruction, to come up with any meaningful formula to explain why one might be safer than the other.