If you have met the revalidation requirements before the end of validity period, you then need to have your licence 'Rating - Certificate of Revalidation' page signed up by an Examiner, together with the appropriate form (download at http://www.caa.co.uk/docs/33/SRG1119.pdf
Send the completed form to the CAA.
There is nothing to pay to the CAA; neither should the Examiner charge you anything for signing your licence.
If you don't meet the 'hours' requirements, you will need to fly a 'revalidation LPC) with an Examiner; if you don't do this before the validity date, then you will need to fly a 'renewal LST' with an Examiner. In either case, he/she can then sign up your licence.
The 'renewal LST' is not
the same as your original PPL Skill Test - it is a much simpler test and has virtually the same format as the 'revalidation LPC'.
All this should have been covered during your Aviation Law and Operational Procedures examination study.......