You have to the CPL ground exams, for either paid or unpaid and you have to enrol with a formal groundschool teaching establishment as they have to endorse your exam application to the CAA. I did mine distance learning about 4 years ago. For CPL I dont think you have to go for 4 weeks in house. I arranged to do a one day in house class for each of the 9 CPL subjects. So it was 9 days over about 15 months in my case... which worked out to be manageable.
It may all change under EASA in 08 or 09, but who knows....
Speaking personally, whilst a lot the CPL theoretical knowledge is widely regarded as being over the top for SEPL PPL instructing, (how jet turbines work, how to navigate from Sydney to Moscow, etc) as an Instructor you are supposed to be able to teach the groundschool courses at PPL level as well as teach the flying lessons to students, so working through the CPL knowledge certainly got me back up to speed in that respect.