I have fairly recently completed my Instructor Rating (October)
So here are my experiences.
I was looking to do it with HeliAir, where I worked, however, they did not have an "Instructor instructor", so it was arranged to do it at High Wycombe with Helicopter Services.
Mike Green was the instructor running the course, who has had many years teaching, as well as his time in the AAC (which he has many good stories to tell).
Also on the course was another pilot of similar experience to my own . This I feel was important, as it aided the learning experience, being able to bounce ideas of each other during lunch, and the ability to fly with each other and practice teaching.
Leon Smith was my examiner, and I found him a very good pilot. There was a lot of banter before and around in the office, but as soon as we got in the cockpit, it was strictly professional.
All in all, truly recommended ,
Best of luck
Andy