Ady,
No, you don't need an ATPL (or even an fATPL) to do this kind of work.
However, when it comes to the written exams, you would be well advised to do that ATPL exams. The reason is that the ATPL exams entitle you to do an Instrument Rating at a later date if you decide to do so, for not much extra effort. On the other hand, if you do the CPL exams, then change your mind in a couple of years time, you'll need to do the IR exams, which will needlessly duplicate a lot of the stuff that you'd learnt and forgotten for the CPL exams.
Good luck!
FFF
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