Hi Gwass,
Its available all year its just that some days are reseved for ground maintenance etc.
Here's the link to the application form, it gives you full details of what days they do the special access.
Unfortunately, the form is only valid up until March this year so I expect the new one will be available pretty soon.
I remember phoning them to make sure when I wanted to go was available. You may also be able to get the latest prices.
Incidently, when we went we took my full DSLR kit, video camera and monopod and nobody batted an eyelid and the security guys were really laid back about it all. Hopefully you will get the same treatment as use.
:o)