SORTED. Thanks BBloke - searching the input field name worked a charm.
So... to manually set the RTE editor,
Within the mutate_content.dynamic.php in the "manager/actions" folder - look on line 1197 and amend the line:
'editor' => $which_editor,
To become:
'editor' => 'The_Name_of_your_hardcoded_RTE',
so, in my case manually setting it to 'TinyMCE4' meant I made it read:
Save and bang - default RTE is set hardcoded.
To get the value name of your RTE - again use firebug to inspect the Manager element of the <option> element where the RTE options are listed - you can then hard code as above.
dubbs.