Ok so I've sussed it! The play station store won't let you buy it because it cannot see you have house of wolves installed. My house of wolves installed automatically, and even though I have it, it wasn't showing up on my add ons. Here's what I did. Go to the options bit on your destiny. Scroll down to add ons. House of wolves will be displayed as "free". Click on this and select download. It will now appear in your add ons and you can now purchase the digital edition for £40.