Philip20004uk,
I did my FAA PPL in 2003. It includes a Night Rating, you need that for a JAA CPL.
I assume you are doing this as part of training towards a JAA fATPL. I hour built in the US at the same time and then after 100 hours total time, I took Air Law, Human Performance and RT exams then took a conversion flight it cost me $300. I now have both a FAA and a JAA PPL and i now have a fATPL.
A friend of mine however did this. He took his FAA PPL, then his ground school and hour building, got his JAA CPL then ME and IR. You only need a ICAO License for CPL issue and unless you are planning to fly here while doing your groundschool, the FAA route seems cheaper and the easier option, less exams. You will need to take your RT exam at some point.
FF