Nick: in JAR-OPS 3 the two senarios you describe are termed Class I and II.
The JAA have tried to avoid confusion with the JAR-27/29 certification definitions for Cat A & B.
The rule does allow the continued use of Class II take offs from oil rigs until 2005 (I think from memory). Sub Part H describes the extra requirements (eg engine usage monitoring [which standard with the HUMS in the North Sea fleet]) if you do have exposure. That time limit could be extended, but that'll depend on EASA. I think the original plan was to base the decision on service experience.
Class II is not an option from city centre heliports - obviously.