Unable to re-download previously purchased music from iTunes

What happens when you try to redownload it ? Is it still in your country's iTunes Store ? If not, or if you've moved countries since buying it, you won't be able to redownload it, you will have to copy it back from your backup of your downloads/computer.

If it's no longer in the store you won't be able to redownload it, and if you don't have a computer you won't have taken a backup copy of it to resync to the device.

I should be due a refund then.

No, you are only guaranteed one download of any item from the store, the store isn't a backup