IHL
An ICAO ATPL-H will be swapped for a JAA CPLH + IR providing that you either do the full 750 hrs ground school & pas ALL the ATP exams + skill test, OR approved ground school as directed by the training provider and pass ALL the CPL & IR exams.
Then you must pass the skill test for CPL & IR.
"However an ICAO ATP will be given JAA ATP, If you meet all the JAA experience requirements for issue, you will be exempt any formal training (ground or flight), provided that he holds a current type rating for the type on which he will complete the skill test and ahs logged 500 hours on that type. If he does not, he must complete the appropriate TRTO course to obtain that type rating.
Similar requirements would apply to the holder of a valid non-UK ATPL/H who wishes to obtain a JAR ATPL/H. However, unless the JAR-FCL skill test includes a multi-engine instrument rating renewal, the applicant will be issued with only a JAR-FCL CPL-H. If he subsquently require an IR-H, he will need to complete a full approved IR course in accordance with JAR-FCL(E)."
I think that if you have a 1000 hours including 500 on the multi engine test ship.
FFI, Check out
http://www.srg.caa.co.uk/documents/srg_fcl_gid26.pdf
Hope that helps