The last thing I'm trying to do is change the text in an particular extra field dependant on stock levels i.e if out of stock it would read "stock arriving soon" and if in stock it would read "In stock".
I would just use the following code to display stock levels on the page:
{if $product.appearance.empty_stock}
<b><p class="message">Stock arriving soon</p></b>
{else} <b><p class="message">In stock</p></b>{/if}
But i'm using a pre-order / back order mod which allows customers to purchase if stock levels are 0 and this in turn makes the website read "in Stock" for back order purchases when actually it's not in stock, so a little confusing for the customer.