Cadfael the JAR requirements for ATPL(H) are somewhat different from those formerly applicable to CAA licence holders.
The JAR ATPL(H) requires 1000 hours TT, 250 hours as PIC, 350 hours multicrew & multiengine plus 70 hours IR time!
Assuming that you meet these requirements your UK CAA ATPL(H) can (in theory at least) be exchanged for its JAR equivilent.
The privilages of a JAR ATPL(H) may (ultimately) be exercised in any of the 32 JAA member states without prior conversion. In practice a JAA licence can only be exercised within a member state which has 'implemented' JAR.
Currently I understand only Denmark, France, Iceland, Sweden, Switzerland and the UK have implemented JAR so that's the likely limit of privilages!
May find more info from BHAB or via
http://www.plh.co.uk