Assuming flungdung is correct (which I am sure he is)
JAR FCL-2.015 applies,
Different requirements if PPL or CPL (strangely enough).
CPL is dealt with in JAR FCL-2.015(c) which basically says you can't
convert unless there is a reciprocal arrangement with a non JAA state , as far as I am aware that doesn't exist with the USA so it can't be done.
You may however be able to validate your license for 1 year based on the arrangements in JAR-FCL 2.015 appendix 1. (Lots of hoops to jump through, which I wont list here).
PPL in Appendix 2 to 2.015 and says
You need an ICAO licence.
You need a JAR class 2 medical as a minimum.
Have more than 100hrs as a helicopter pilot.
Pass Air Law and Human performance exams.
Relevant bits of subpart F which i presume means 2.240.
Pass a flight test for initial PPL issue.
Hold an acceptable RT licence (which means usually passing the RT practical and written exams, and applying for the licence).
JAR FCL-2 Section 1 can be found at
http://www.jaa.nl/section1/jars/444077.pdf
The pages you want are
JAR FCL-2.015(a-d) and 2.016 pages 5 & 6
Appendix 1 pages 18 & 19
Appendix 2 page 20.
It is of course possible to apply for a JAA licence if you hold an FAA licence and your hours will count towards it at least JAR FCL2.016 applies in this case and it is not a 'conversion'. Simply stated.
Have a Class 1 Medical.
Training as required to pass 'the equivalent' JAA knowledge exams.
Flight training as required at the discretion of an FTOs head of Training.
Pass the Flight Test.
Hold an RT Licence.
All explained in LASORS 2005 available at
http://www.caa.co.uk/docs/33/LASORS2005.PDF
If you need any more details let me know !
Cheers
V.