I had a similar problem in a P reg 2L auto. It turned out to be a burned out solenoid in the gearbox. I can't remember exactly how much it cost but it was about �100.
There is a bit more to the story. I initially went to a Renault dealer, they charged me �35 to do a diagnostic and told me I needed a new gearbox at �1600 without saying what the fault was. I then went to an independent Renault/Peugot specialist who did a free diagnostic and told me it was a solenoid. He then said he would remove the solenoid pack and attempt to repair the broken one, warning me that if he couldn't fix it, he would have to replace the whole solenoid pack at a cost of around �100, which is what eventually happened. So I recommend a Google search for non-dealer, Renault specialists. This was a few years ago so prices will have gone up. And, of course, your problem may be completely different.