I just posted re this, but am not sure whether it went

Hi,
I just posted re this, but am not sure whether it went to forum or to Support; so I'm reposting in hope that someone in WPMU support sees this..
Basically, I've network activated New Blog Templates (NBT) and have created and modified a subsite to function solely as a template. It has blog ID 'Blog5'. But when I try to create a template in NBT I keep getting error message taht I cannot create a template from main site/blog. But it's NOT main site, but is a subsite! So...what am I doing wrong, please?
Happy to create a user with creds if you need to see back-end of network.
Cheers,
Paul