You will need to fly a SEP Class Rating renewal LST with a Flight Examiner. Nowadays this also includes some navigation; however, the actual navigation content is left to the Examiner's discretion having taken into account your recent flying. So, do some refresher training, then the renewal LST. Your Rating will then be valid for 2 years from the date of test and you can use it as soon as the FE signs the certificate in your licence.
You won't need to resit any exams; neither will you need to pay anything to the CAA. Just the Examiner's fee and cost of a/c hire.
Check the front of your licence; if you passed in Oct 2002, it will need to be re-issued before Oct 2007. To do that, you must have a current SEP Class Rating and medical, complete a form SRG1102 (available on line)...........and pay the extortionate re-issue fee (the Belgrano call it a renewal fee - it isn't, they don't even use their own terminology correctly
), which is currently £63.