According to the CAA:
Until such time as EC Directive 91/670/EEC is withdrawn, a UK National licence can still be validated in other EU States.
If your PPL was issued in 1999, it is a 'lifetime' UK National licence. It would thus seem that all you need to do is to pass the SEP Class Rating Licensing Skill Test with a Flight Examiner authorised to do so. Such as a PPL Examiner authorised by the IAA. He/she can then sign your Certificate of Revalidation and off you go.
But you would be best advised to check this with the UK CAA first.
Any views, Whopity?