Hmmm - looking at this some more, it seems you cannot put a template into a sub-folder if you are calling it yourself from a context, because its in the calling the template that the script Katherine refers to seems to be called
However, if you are overriding an existing template e.g. the main.mustache tempalte for the new myOverview block then that does need to be in a sub-folder structure (as it is in Boost)
So, if you are extending it and changing the context and therefore having to call the template yourself, then it appears to be as Katherine already said. But if you are simply re-arranging the existing template, using the existing call and context, then it goes in a subfolder structure representing where the original template that is being cloned came from.
Hmmm - seems overly complicated to me. We obviously need the subfolder structure if you are overriding one (there may be many blocks with a 'main.mustache' file, but if I want to change the context for several of those, we would have to rename them as well, rather than put them in a subfolder. Surely for ease of development and for consistency, subfolders should be allowed to enable this kind of development more easily?
(or have I misunderstood the way this works???)