The resolution to the problem is to set-up the SQL Agent job step as a Operating system (CmdExec) and in the command line use a statement similar to the one below:

C:\Program Files (x86)\Microsoft SQL Server\90\DTS\Binn\DTExec.exe /f "f:\bi\64bit\DUMP\DUMP_DATABASE\DUMP Database.dtsx" /MAXCONCURRENT " -1 " /CHECKPOINTING OFF /REPORTING E /DECRYPT xyz123

The 'xyz123' after the /DECRYPT clause is the password used when saving any sensistive data in the package e.g. database passwords.

there doesn't appear to be any explanation on MSDN as to why you cannot use a Job Step type of: SQL Server Integration Services Package

to execute the package. This only appears to be an issue with SQL Server 2005 64bit