Instead of my actual address I had something different coming up..
Safari uses information found in your contacts (if you let it). In Safari Preferences, you can click Edit next to 'Using info from my contacts' and it will open up the Contacts App with your details already selected so you can update them.
Alternatively you can open the Contacts App manually and search for yourself in it. Once you find your contact (the one with the user icon next to it) you can update it to reflect the information you want autofill to use.
Once you update these details, Safari will start using them for its autofill feature.