Wodpress 4.1 - blogtitle missing from mp_tag and mp_category pages

This issue was addressed and fixed very kindly (see URL below), when title was set in header.php. With the advent of WordPress 4.1, our theme no longer sets the title in header.php, so we have again lost the company name in the page title of mp_tag and mp_category pages. I believe this modification must be put into a function using remove_filter and add_filter. I tried modifying the original fix for use in our functions.php to no avail.

Any assistance would be very much appreciated.

The original fix for the header.php is here:

Any tips or solutions would once again be greatly appreciated.