This is driving me nuts!

I have a requirement to add several page templates, installed through a package and would like to edit the pages using them through composer. However, when I create a new page from the page type using the template, the composer is saying "Unable to load block into composer. You must edit this content from within the context of the page."

Before you ask, yes I added a composer control (body) to the output in the 'Page Type' section!!

This is a 'text-book' vanilla install of concrete5 5.7.4, and even the template is actually using a copy default.php so its the same as a full-page. When creating page that uses the full-page template, ironically, it actually works! So what have I missed?

I saw in a discussion on github:https://github.com/concrete5/concrete5/issues/1739... back in January (2015) including Andrew Embler describing the problem, which seems to have been around for ages - come on guys when will this get sorted??

Looking to any help that can be provided please....