whirlybird,
to instruct and be paid, you have to hold an actual physical CPL licence, have got 300 hours total flight time,of which at least 100 must be PIC, and then you can do the 30 hour FI course.
to instruct and not be paid, you can add an FI rating to a PPL as long as you have 200 hours PIC before you do the FI course, however, also you must have at least passed all the CPL ground exams, so I guess in your case the first option applies as you want to be paid.
let me know if does not make sense compared to what others have told you and I will try to resolve it.
it is my opinion that too many people give out info based on hearsay who have never even seen a copy of JAR FCL2 let alone actually looked inside it, and this leads to a lot of confusing misinformation.
it will probably turn out that I have got it wrong now, but I dont believe so based on the background reading I have done.
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elpirata