SEP is used to define a licensing category.
JAR-FCL 1 Subpart F defines Class Ratings:
JAR-FCL 1.215 Class ratings shall be established for single pilot aeroplanes not requiring a Type rating as follows:
(1) all single engine aeroplanes (This is your SEP, no upper or lower weight limit!)
Therefore, if you have a SEP Class rating in your licence you can fly any single engine aeroplane that does not require a Type Rating.
There is one proviso, JAR-FCL 1.005 (a) (5) Whenever a reference is made to aeroplanes this does not include microlights as defined nationally unless otherwise stated. So you can fly one but cannot claim the hours towards SEP revalidation.