Not able to add new sub-templates - error message

Hi

I'm trying to add some additional sub-templates to our Xerte 3.11 install (on a test server and I've tried on an old 3.10 test version too) and I keep getting this error: Parent template 'Nottingham' is not recognised as a valid parent template in this installation.

We already have two sub-templates installed (one under Site and one under Xerte). The Nottingham template is present and active.

Has anyone come across this before?

Cheers
Yvonne