Congratulations on obtaining your PPL!
This is a licence which contains (probably) a Single Engine Piston (Land) Class Rating.
The Rating validity may be maintained either by 'experience' or by 'test'. To maintain it by 'experience', in the SECOND 12 month period, you need to fly 12 hours TT, of which 6 must be as Pilot-in-Command, plus 12 take-offs and landings. The 12 hours must also include a 1 hour training flight with an authorised instructor - this is NOT a test! As soon as you've done all this, your Rating may be re-validated by an authorised Examiner who will sign your licence Certificate of Revalidation page - no fee involved.
Alternatively, you may revalidate by 'test'. This means flying a simple Proficiency Check with an Examiner in the last 3 months of your Rating validity period - about an hour with a mandatory test content.
But remember that, notwithstanding the Rating validation requirements, if you want to carry passengers then you must have flown 3 take-offs and landings as 'sole manipulator of the controls' in a/c of the same 'class' in the previous 90 days.
In addition, the licence itself must be re-issued every 5 years; for this you must complete the relevant form and send off the appropriate documents to the CAA, together with the re-issue fee.
Really, all this should have been covered in your Air Law ground training. You can also find it all in LASORS, downloadable for free from the CAA website.