To convert an ICAO PPL to JAA PPL, you need the following:
A current ICAO PPL (temporary license are not acceptable)
Current medical (same State where ICAO PPL is issued)
Current JAA medical (minimum class II)
If less then 100 hours flight time, all seven written exams
If more then 100 hours flight time, three written exams (Air Law, HPL and RT)
If applying for conversion with UK CAA- RT Practical (FCL Form508)
PPL Skill test with UK/JAA FE
Applicable fees (I believe it is £143 at the moment for PPL and £64 for night)
You are not required to take any flight further flight training to take the JAA PPL skill test, but personally I think it is a good idea to take a couple lessons.
I hope this helps.
Jatin