Been searching the whole CAA site and still couldn't find the answer!
Try downloading LASORS (which is free!)
anyway, question is, i know that the PPL is valid for 2 years but what makes myself current.
A PPL(A) is issued for 5 years. Ratings contained in it have various revalidation requirements. The SEP Class Rating has a 2 year validity period; to revalidate it 'by experience' you need 12 hours (6 as PIC) in the last 12 months of the validity period - and 1 of those hours must be a training flight with an authorised instructor. Or you can choose to revalidate 'by test' by flying a Revalidation Licence Proficiency Check with an Examiner in the last 3 months of the Rating validity period.
i know that i need 12 hours in the 2 years but do i need 1 hr every 90 days?
The 12 hours has to achieved in the second 12 month period, not over 2 years.
You have to have flown 3 take-offs and landings in the 90 days before the date of your intended flight if you wish to carry passengers. One of those must have been by night if you want to carry passengers at night (assuming, of course, that you have a night qualification). There is no recency rule as regards flight without passengers; clubs offering aircraft for hire may well have their own local requirements, however.