In a nutshell,
If you have 500 hours on a multi pilot (eg, Shorts 360, Dash 8, 737) aeroplane then you don't have to do the groundschool or the training to do the check. You just have to do the check with a TRE and a CAA examiner.
If you have 500 hours (or more) in multi crew operations but NOT ON a multi pilot aeroplane. (Eg, time is on a Bandit) then you have to do a Type rating course with a TRTO, then do the check with a TRE and a CAA examiner.
You don't need 500 hours on it IF you do a TRTO course.
My Dash hours contributed to my over all multi crew time but since I didn't have 500 hours on it, I would have had to do a full type rating. I think about 15-17K.
Please note the difference between multi crew and multi pilot.