I'd be interested to know more about this position. Once a driver, there's not much scope for progressing your career at my depot. The instructor positions seem to be taken up by instructors who have no intention on moving on, and all other depot or company training centres are too far away to commute.
I do wonder if its more to do with the training of train drivers in rules, classroom based traction and other training courses (driver instructor courses for instance). Once traniee drivers are competent in rules the handling would be done by the TOCs still (not sure if I explained that very well!). My TOC currently use an outside company to train mentors and instructors, so it seems it could be another one of these companies.
It seems they also have a number of guards and drivers they can hire out as almost like agency staff. I guess they hope that if TOC's go on strike, they'd be called upon. They strike me as a company like ROG.
I'll be emailing them later to ask for more information. My major concern is how you'd keep route and traction knowledge up. Also, if there's a lot of working away and staying at hotels, I don't think it would be worth the extra £100ish a month to be away from my family to do the job.