John
You do not need a CPL to take your FI but you do need the 200 hours and having passed the CPL theory exams. That said you will need a minimum of a CPL if you wish to be paid for your instructing.
You will need to take a pre-entry flight test before starting the instructor course, and have completed at least 5 hours flying in the 6 months before that test. Also you will need to have completed 20 hours of cross country flight including at least one long (540km XC).
There is also no official requirement for a night rating although I cant remember hearing of an instructor who didn't have one (certainly not in my neck of the woods).
RIX