Hi there,
I want to nest multiple projects into a single plan - to manage resources.  I have made a template plan which lists core tasks and saved it.  I copied the template plan three times, renamed the files, and then opened and updated each of the files - as there were additional tasks specific to each plan.
I opened the master project plan and imported the project plans successfully.  However, the title for each imported project plan was not the file name, nor the title on line 1 in the plan.  It was instead the original file name, which I used to create a template plan from!.  
If I open the activities of each imported plan, I have the title of the project on the line below the file name..
I don't want to have to set up new plans for each project, but have found that if I copy the title and tasks of a plan inot a new template it imports without any problems..
Any thoughts please?

Try Tools > Options > View-tab and put a checkmark in "Show Project Summary Task".  Then edit the name in Task 0.

I don't understand, though, why File > Properties and changing the entry in "Name" (and then saving the file) didn't work for you.