Hello I've recently just bought a new iPhone and i am wanting to transfer all my music that i habe bought from my old one onto the new one. I've gone onto iTunes and tried to download them all again but it wont let me?

What happens when you try to redownload them ? What you can re-download (the types of content that you can redownload varies by country) will show under the Purchased link under Quicklinks on the right-hand side of the iTunes store home page on your computer's iTunes, and the Purchased tab in the iTunes store app on your iPhone (and similarly for apps in the App Store app). If you still have your old phone then you should be able to copy your iTunes downloads from it over to your computer's iTunes via File > Devices > Transfer Purchases