I think you had to have 1000 hours P1 before the CAA CPL (H) could be converted to a CAA ATPL (H), presuming all the necessary CAA written exams were passed. Therefore if you have a JAA CPL(H) and 1000 hours P1 it is the same as the minimum old CAA ATPL (H). To have a JAA ATPL (H) you need an IR in addition to the required hours in their various forms.
A JAA CPL (H) includes all written exams necessary for award of a JAA ATPL (H). In other words the holder of a JAA CPL (H) does not have to take further written exams to gain an IR and subsequent conversion of his CPL (H) + IR to ATPL is dependent solely on completing the required hours.
I think that's it anyway.