You need to fly a renewal proficiency check with a PPL Examiner. It will take about an hour and consists of the usual steep turns/stalling/PFL/circuits etc. It will cost you whatever the Examiner charges, plus the ac hire. No fees to pay to the CAA, but the form goes off to them even though the Examiner should be able to sign you up in your licence (not logbook) there and then straight after the flight.
For full details, go to
www.srg.caa.co.uk and look for GID33 'Keeping your licence and rating current'.