Steven B
The validity of your licence commences from the date you passed the Skill Test. The rating page on the licence should contain a valid until date, 24 months from your Skill Test. That is the date by which you must revalidate the licence or it expires on that date.
As the others have said if you wish to revalidate by experience, then 12 hours in the 12 months prior to the expiry date is required. You can revalidate at any time in the last 12 months, don't wait until the date, you get another two years which runs from the initial expiry date, no time is lost.
Remember 12 hours is only one option, you can do a proficiency check in the 3 months prior to expiry and no other experience is required.
Flights with non JAA instructors don't count for revalidation by experience. Its up to you to prove the landings, so log 12!