TooHotToFly - do you mean the PPL Skill Test or the Single Engine (Piston) Land Class Rating Skill Test?
PPL Skill Test - includes the en-route section and slightly more General Handling than SEP (Land) Class Rating Skill Test. Form FCL 684 would be used for the PPL Skill Test. The SEP (Land) Class Rating requires certain items to be covered during training, others are also covered in the test itself; form LST SPA is used. The SEP (Land) Class Rating would be appropriate if you were adding it to a PPL with TMG rating, for example.
The SEP (Land) Class Rating is either revalidated by completion of the appropriate requirements (which might include a Revalidation Proficiency Check), or, if lapsed by less than 5 years, renewed by completing a Renewal Proficiency Check - the contents of both checks are identical. If lapsed by more than 5 years, the CAA may specify additional training as well as the Renewal Proficiency Check. In all cases, form LPC SPA would be used.