I had the exact same query a few months ago and got the following definitive reply from those nice people who got me through the ATPL GS.
From JAR FCL 1:
Appendix 1 to JAR–FCL 1.160 & 1.165(a)(4) CPL(A) modular course
(See JAR–FCL 1.125(c)) (See JAR–FCL 1.160, 1.165 & 1.170) (See Appendix 1 and 2 to JAR–FCL 1.170) (See AMC FCL 1.160 & 1.165(a)(4)) (See Appendix 1 to JAR-FCL 1.470) (See IEM-FCL 1.170)
1. The aim of the CPL(A) modular course is to train PPL(A) holders to the level of proficiency necessary for the issue of a CPL(A).
2. (a) Before commencing a CPL(A) modular course an applicant shall be the holder of a PPL(A) issued in accordance with ICAO Annex 1;
(b) Before commencing the flight training an applicant shall:
(i) have completed 150 hours flight time as a pilot; and
(ii) have complied with JAR–FCL 1.225 and 1.240 if a multi-engine aeroplane is to be used on the skill test.
In english, what it all boils down to is that you need 150 hours total. However, individual FTO's are allowed to insist that of those 150hrs, a number of them must be P1 - this is not a JAR requirement. I guess its to protect the aircraft and instructor's of the FTO who dont want to fly with students who have low P1 hours. The school I am with looked for 70 P1 while others look for 100. I havent come across anyone who will take you with less than 70. Everything else said above is accurate - you can sit the skills test with less than 200/100 but you cant get license issue until you do.