[AndrewBrown]

as you say it could be a sticky or intermittent gearbox inhibitor switch,

The switch prevents you from starting the engine in gear I think its on the base of the selector lever

it prevents the engine from cranking over if in gear if I remember correctly, so if it does crank over it probably is not that, but I'm not sure if it cuts the out ignition circuit out too, if it does it could still be faulty by cutting the ignition out and not the starter motor or the other way around,
either way Id look at that first but also those Renault engines did suffer from condensation build up inside the distributor cap a lot of the time preventing starting.