I can't understand what's taking all my storage as the apps listed don't add up and I don't download videos or music etc. My iPad says I've used 12.2GB with 209MB available yet when I added everything up my total usage was 8GB. CONFUSED
Most likely it's the operating system, in this case, iOS. The operating system needs a certain amount of space on the iPad.
In this way, you don't get the amount of storage that you pay for, since iOS tends to usually take up anywhere from 1 to 4 GB of storage, depending on which version you're running.
This issue is a little strange. Have you checked your "Recently deleted" album? Maybe it takes up your storage. The pics and videos you deleted are not actually gone. If you want to make them disappeared you need to go to the "Recently deleted" album and delete them again. Otherwise you need to wait for 30 days they will be deleted automatically. You should check it.