The NPPL FAQ at
http://www.nppl.uk.com/faq.asp says:
Q Can I upgrade to a JAR-FCL PPL?
A Yes. 30 hours can be carried over as a credit towards a JAR-FCL PPL, as long as all flying training has been completed by a JAR qualified instructor and all JAR PPL exams have been passed. 15 hours of training will need to be completed, of which 5 can be done on a simulator.
And dont worry, this is the right bit of PPRuNe
Hope this helps