Jobless,
This might help you.
I too am doing CPL and have checked with the CAA on this subject.
I understand that the CPL exam questions are drawn from the same database as the ATPL questions
there is only the one database).
In effect they are the same questions, except that they do not cover such issues as IFR, Mach numbers and jet engines, etc that rightfully belong to the ATPL.
If you have not already got them, I suggest you get the ATPL/CPL/IR learning objectives from the JAA website. From these, you can see just what is in the ATPL that is not in the CPL.
Very best wishes,
Broomstick