Assuming you are talking about a modular course rather than integrated, you need to have 155 hours before you can start the CPL(H) flying course. The course is then lasts 30 hours or 35 if you don't have a Night Qualification.
You can either do CPL exams (9 in total) or ATPL exams (13 in total). An ATPL is a separate licence with stringent requirement, the ATPL exams do not an ATPL make!!! If you don not get an Instrument Rating within 3 years, the ATPL exam credit become invalid and you would have to sit the 7 IR exams again.
You have to have passed the exams before you take the CPL skills test.
Have a read of the "Becoming a Helicopter Pilot" sticky thread above and check out LASORS from the CAA website.
Cheers
Whirls