Menu Item Overrides Joomla 1.7

[hnje6482]

Hi there

I am very confused about menu overrides. I decided to create a new Category Blog layout. I did the following:

copied blog.php, blog.xml, blog-items.php, blog_children.php, blog_links.php to
<template>/html/com_content/category

and renamed them to exblog.php, exblog.xml, exblog_items.php, exblog_children.php and exblog_links.php (respectively).

I added three lines to en-GB.tpl_<template>.sys.ini as follows:

TPL_<template>_COM_CONTENT_CATEGORY_VIEW_PPBLOG_TITLE="Portfolio Layout"
TPL_<template>_COM_CONTENT_CATEGORY_VIEW_PPBLOG_OPTION="PP Porfolio"
TPL_<template>_COM_CONTENT_CATEGORY_VIEW_PPBLOG_DESC="Portfolio Layout"

I see the menu item when I create a new menu item. However, it is not calling exblog.php.
If I add blog.php to <template>/html/com_content/category, it picks that up fine but not my exblog.php.

What have I done wrong ?