thanks for replies so far, please keep them coming!
Yes, it was the R22 I trained in. The schools' explanation for the 20 hour limit was twofold:
1 For insurance purposes, as stated above
2 To take the pressure off the student
Knowing that there is no way you are allowed to solo before 20 hours goes some way to preventing the "your w***y is bigger than mine" scenario.
This is so much better than the fixed wing scene, where, (even though it is debated), the time to solo can (and does), become a competitive issue.
Also, I know a rotary instructor who didn't reach the solo stage themselves for 30+ hours - this person, (IMHO), is a superb instructor.
Just goes to show....