As I am returning to this industry after 15 years away from it and getting used to the digital tacho is rapidly becoming the most worrying aspect of it all (originally it was passing the medical although I actually passed this with flying colours 
) especially as there seem to be 3 different types out there.
This thread has proved to be very helpful and I thank the OP and all the other posters kind enough to give there time to share their invaluable wisdom.
I shall read and reread all your comments in readiness for that moment in a few days time when I climb up behind the wheel look up at whichever model I find installed and hopefully all your advice will come flooding back.