Well, I drafted the top-up requirements, the CAA agreed that they met JAR-FCL requirements and they were later published in LASORS.
NPPL(SSEA) to JAR-FCL PPL(A) top-up requirements have stood the test of time unchanged for quite a few years now - but again, it is clear that very few flying schools are aware of the requirements. It seems that either it's all too difficult for them, or they can't be bothered to market any top-up training.