Ok, after experimenting all is now clear.
In Dave's instructions there is a step in which you 'Convert' an existing subsite to a new type - the type that you have just created the package to define.
The option will appear under 'Main Site Administration' at ./admin/ once your new subsite 'type' has been installed. This would seem to be analogous to sub-classing the acs-subsite package, and once a new sub-class is installed and available, the option to change an existing subsite to be of this new type is presented.
The word 'dependent' appears in the above instructions, which confused me when initially reading through. The option to select is actually:
"Convert to Descendent Subsite type".
Descendent in this context makes more sense.
I hope this helps someone.
Regards
Richard
