The full PPL Skill Test is only ever needed once. It includes an en-route section and various GH exercises.
The re-validation Proficiency Check is flown to revalidate a rating for those who cannot meet the experience requirements. For SEP, there is no en-route section.
Now comes the difficult bit. Originally, it was simple - valid ratings could be revalidated by a Proficiency Check, lapsed needed renewal by Skill Test. But the CAA cocked things up hugely by introducing a Renewal Proficiency Check for lapsed SEP ratings contrary to everything they'd told us in the pre-JAR roadshows. There then followed some rapid weasel-wording in which they announced that 'To renew a lapsed SEP Class Rating, an acceptable demonstration of Skill will be achieved by passing a Proficiency Check'. Don't worry about it - it's exactly the same test as the revalidation Proficiency Check - it just means that if your rating has lapsed you can't fly as Commander until you've passed the Proficiency Check. It won't cost any more than the re-validation Proficiency Check - and you won't need to pay anything to the CAA unless your rating had lapsed by more than 5 years.
The only other variation affects people who have a licence without an SEP rating - such as the odd military ME pilot with an ATPL or a PPL holder who has a TMG rating. If such folk want to fly a SEP aircraft, then they have to fly a SEP Class Rating Skill Test which is not the same as a PPL Skill Test.......
There is NO mandatory content for the '1 hr training flight'; furthermore, it can be replaced by any other check or test for a JAR licence or rating - or an IMC rating test. Hence an IMC revalidation test, an airline OPC in the simulator or a military flight with a military QFI as part of Service duties will do fine - as would your MEP revalidation Proficiency Check.
Confusing? Not really - just cumbersome Eurocracy!