ree.t
I can comment on the IAM course since I taking it at present.
The group I am with (East Lancs) considers that a 12 week course is suitable but will provide observers for as long as necessary provided that the candidate plays his part nad is willing to learn. Each group apparently has its own methods but East Lancs provides you with a one-to-one observer for one drive per fortnight and a random observer each Sunday in the intervening weeks.
To be fair I seem to be stuck at the 6 week point since I have great difficulty in removing bad habits gained in 30+ years of driving.
The course ends in a driving test carried out by a Police Class 1 driver (they seem to know their stuff

). If you are recommended then you can look forward to receiving your certificate and an invitation to train as an observer.
The IAM makes extensive use of volunteers and may well be the least expensive route to an advanced driving qualification.
Regards
Malc