in the Joomla admin

• Extensions --> Language Manager
• Select the language you want to alter i.e. English (en-GB) (you might not need to do this but it does not harm)
• Click Overides
• Click New
• Change 'Search For' to search Constant
• paste or type JPAGETITLE
• Click on the JPAGETITLE highlighted area that appears below
• Replace '-' with '|'
• Click Save and Close

The '-' is now overided for '|' for that language only and with no core file alterations. This is also only for the Frontend.

Links

• joomla3.0 - Change Joomla 3 page title and site name separator from '-' to '|' - Stack Overflow