Ok, a quick update following support calls to Symantec & Microsoft;
Microsoft confirms that SQL 2005 & 2008 will not co-exist on a server.
Symantec confirms that BE12.5 will not run on 2008
BE2010 - will work on 2008, but as the installation of 12.5 SQL DB will not start, when we try to upgrade to 2010 it is highly likely to corrupt the BE database.
They recommended manually creating an DB instance on 2008 called "BKUPEXEC" and then during the upgrade to 2010, manually point it to that instance. If this should fail, then a full removal of 2010 & 12.5 including registry clean would be required before attempting to re-install again, but again using the manually created SQL 2008 instance.
I'll provide further information as we attempt this...