So, though a little bit of delving into the code meant that I've found a solution to this. And seen as google was no help, hopefully this will be of help to someone else.
My code now looks like this
if( $this->getSession()->getDriver() instanceof Behat\Mink\Driver\Selenium2Driver){
// Selenium Code
}else{
//Goutte Code
}
I've just grabbed the driver object and have checked which driver class it's an extension of, simple.
Now I can run the same step definition if the @javascript
tag is or isn't before my scenario.