Hey Simon!
My name's Alex - I'm a Guru here with Shopify.
There's an app in our App Store that will let you set up free items for both buying a particular item and for buying a certain amount in £. It's called Unlimited Upsell: https://apps.shopify.com/unlimited-upsell
It also lets you track sales based on that promotion so you can see whether or not it's effective.
Hope that helps!
Have a good day!
-Alex M.
Shopify Guru
alex.missett@shopify.com