What is wrong with the fluorescent tube light in my loft?

Florescent lights have a ballast that controls the power to the light and it needs to be replaced.

Remove the little small round ballast and replace.

You must need a new fitting.

I agree with your best answer, that the ballast is the issue. You can replace the ballast for about $10, but another option is to remove and bypass the ballast, and then install LED fluorescent bulbs instead. You can get two for $20 or less, and they'll use much less energy than standard fluorescents, they'll generate virtually no heat, and they'll likely be brighter as well. They don't need a ballast or a starter capacitor, and they last virtually forever.

Replace the ballast with a T-8 type, then connect the wires to the tube holders. Then use LED fluorescent replacement tubes, and you will never need to mess with it again!

Fart on it for good measure

It could be the tube is knackered or the starter.

Try replacing the starter...if that.doesn't work, change the tube.

Edit...as you'e tried this, you might need a new fitting.