Having done exactly that...
1) You must pass JAA-CPL(H) or ATPL(H) theory exams
2) You will need a min of 185 hrs before a licence will be issued (i.e. 155 + 30 hr modular course - if you already have 185 no problem but if you have less you'll need to take it into account)
3) You do NOT have to do a modular course - ONLY training as required, however well recommended if you can afford it as it will ensure you're better equipped for the test
4) Check LASORS for mins. of: Night, X-Country; P1 and Instrument (I forget now but I think 5 hrs night, 25 x-country; 100 P1 needed for night rating and 10 hrs instrument)
Get a good school and you're laughing ... I ended up doing almost the entire modular course (in abbreviated time) and thought it highly beneficial (I also had an excellent instructor).
Did my exams through Bristol Groundschool and can HIGHLY recommend (sorry moderators but I don't work for them).
Don't forget all the additional expense beyond the training ... 170A (pre cpl-test), test day alone was nearly £1k ! Groundschool approx. £2k etc
Best of luck,
FO