Scatygirl - you need to apply to the Speedway Control Bureau, ACU House, Wood Street, Rugby, CV21 2YX.
The new regulations do state that you can have a 500 licence at 14 but you must complete a competency test or an assessment course if you haven't already done one.
As I understand it if you are already licensed for a 500 then you will continue to be so regardless of age.