You probably know this, but just in case you haven't read the manual. I used to drive an R-reg Clio 1200cc by the way. It has a peculiar arrangement for the immobiliser.
When you "plip" the key, open the door and climb in, you have about 30 seconds to start the engine before the immobiliser cuts in. If it does, you need to plip the key at the sensor (next to the rear-view mirror) to lock and unlock the doors again. This will deactivate the immobiliser again and allow you to start the engine. You MUST plip the key in order to start the engine, using the key in the door then trying to start the car won't work.
Also, I found that the key was in such a position as to catch on my right knee sometimes and cut out the engine... wouldn't happen when starting it I guess.
