There are loads of articles and posts about doing this in previous versions using several different URL strings, all of which none work in Magento 1.8
Has anyone actually figured out a way to do it yet?
For example, this is listed on the Magento Wiki and was added in 2010: http://www.magentocommerce.com/wiki/4_-_themes_and_template_customization/catalog/adding_a_product_to_the_cart_via_querystring
However when clicked, this link just refreshes the cart.
Just incase anyone wants to see how I'm trying to code it:
<button type="button" title="Add Loan Phone" class="button" onclick="setLocation('URL HERE')"><span class="green">Add a Loan Phone (+£10)</span></button>