Hello @Cazwilkins! This is how it works, you click on 'submit', pay, and then for maybe a few hours you will still not be prompted to add your evidence, there probably even be a message "No documents required at this stage for this application", or something. It is weird, but don't panic.
Just submit your application, then you will get the option to attach evidence. Just attach everything you need to methodically, don't rush. If you forgot something, simply attach that additional info as soon as you remember. Remember that you can attach documents all the way until the day your file is picked up for assessment. So please don't worry! We all have been there.