What is the best product to restore the clearness of dull headlight glass?

What is the best product/method to restore the clearness of dull headlight glass?
Turtle Wax offer a good value cleaner and sealant for about £7: www.halfords.com/motoring/car-cleaning/shampoo-pol...l

Meguiar's offer a more expensive kit for £22, but it includes extras like a easy-buff pad: www.halfords.com/motoring/car-cleaning/clay-bars-d...t
Answered by Georgia Petrie

Ask Honest John