Hi is it possible on magento natively to run the below offer:
Spend over £80 (excluding shipping) and get free t-shirt? So if someone puts 2 items in the cart totalling 80gbp I would like them to see an option to chose a size in a free t-shirt. I know there are probably extensions that can do this but does magento do this natively?
Magento does not come with this feature out of the box. You can use customization or a ready extension. Check out this one https://amasty.com/auto-add-promo-items.html
Its not a native Magento feature and this requires an extension.
An alternative to the extension proposed can be this one, and it does what you are looking for : http://www.magentocommerce.com/magento-connect/j2t-auto-add-va.html