Well, I found my own solution to this - although it's somewhat extreme, and not one I'd recommend!
1. Backup all content you want to keep to the desktop software
2. Create a new MyWorld account
3. Perform a factory reset
Like I say - extreme, but if you really want to do it then it does work. Aaaannndddd, since apps, pictures and music tend to be stored on the memory card as opposed to the phone memory these remain in tact - so long as that's where they're stored!