I recently queried all this revalidation process with one of the NPPL top dogs, mainly the requirement that the 1 hr with an instructor had to be in the last 12mths.
Since 31 Jan 2008 when the ANO was amended the NPPL requirement changed. Revalidation can be conducted at any time in the 24 months of validity and all requirements can be met at any time within that period. It all hinges around the Specified date i.e. the date the signing takes place. The hour with an instructor can be an aggregate of one hour and flown at any time within the 24 month period. BUT THAT IS ONLY FOR THE NPPL
For a JAA licence or a UK licence with JAA rating, the flight with an instructor has to be a minimum of a 1 hour flight (singular) and in the last 12 months of the 24.