My Reports and Report Builder permissions

Hi all
Hoping someone can help with this. The scenario I want to achieve is as follows:

Using SSRS 2008 R2.
Reports reside in various folders, which users have access to by way of AD group membership.
I need certain users (in a 'ReportBuilder' group) to be able to edit reports from a particular folder, but not be able to save over the Master report but instead be able to save a copy to their 'My Reports' folder and edit that.

This is complicated by the fact that out of 2 test users, one can access their My Reports folder and the other cannot, when AD group membership is the same!

At the moment at the top level (site-wide) the AD ReportReader group has the Browser role, and the ReportBuilder AD group has the My Reports and Report Builder roles.

Thanks for any help in advance

Jim