Renault Megane starting problems

If you have replaced battery and it still barely turns over , it could be a "lazy " starter motor . You could check all battery terminals and earth connections .Was it ok prior to servicing / cambelt replacement ?

A poor or intermittent earth would do this. Having had various Renaults through the 80's and 90's I can say that electrickery is not their strong suite.
Anyway, I would temporarily rig a new engine earth and see if the car starts more reliably. If it does, then replace the original. If the fault started after the cambelt change, the original earth may have been missed. I have seen earth straps which look fine, but are corrroded to nothing internally.
The easiest way rig a temp connection is to use a jump lead, and connect between good clean metal on the engine, near or even on the starter motor. (I use a starter motor retaining bolt if I can.) and directly to the negative terminal on the battery. You can connect to a bare metal surfacre on the body, but that's second best. Make sure the cable, and yourself, isn't going to get caught in anything that whirrs.
Now try starting.