OK, I've created a session value in the way you've suggested and called it 'returnurl'. I've give it a value of the page I want to be the default redirect page. I've set the trigger as 'If Session Variable Is Not Defined'.
I've added this to the hidden field like this:
<input name="return" type="hidden" id="return" value="<?php echo $_SESSION['returnurl']; ?>" />
Can you advise how I would go about changing the value of the variable if a specific item is added to the cart?