What causes haloes and blurred vision?

Depends. Your age, past eye history and an exam would help with the answer. If you want a list: cataracts, acephalic migraines, refractive surgery, bad contact lenses, corneal edema, iritis, posterior capsule opacification are a few on the list.
Beware of glaucoma. There are literally thousands of potential causes of blurred vision, but not so many for halos. If what you see is like looking at a streetlight through a frosted window, that is a true halo. It is often caused by water mist between you and the light. It can be seen for a while following eye surgery such as lasik. It can also be caused by acute glaucoma, so your eyes should be checked.