To convert ATPL(A) theory to ATPL(H) theory requires:
Performance
Operational Procedures
Principles of Flight
Composite Paper (100 extra general questions)
I would recommend
bristol.gs for this.
CPL(A) to CPL(H):
135hrs Rotary Total Time
50hrs Rotary P1
I don't know of any specific courses running for this - so whichever CPL provider you prefer.
Regarding converting CPL(A)/IR to CPL(H)/IR. You would be much more employable with the IR and it would theoretically be cheaper to convert from IR(A), however I get the impression that most North Sea Companies will pay for you IR if you are the right applicant. Having said that, I know that some people have been employed the same day that they passed their IR(H).
Bobby