So, it seems the only way to achieve this is to import an email template in simple HTML rather than use the Eloqua WYSIWYG editor.
Now, I love all the things Eloqua can do ...
... but to be honest, I'm a little bit shocked that the leading marketing automation solution does not have an email editor that's capable of creating a responsive designed email for Android devices (>50% of the market).
Definitely one for the 'to do' list!!!