You need to get a CLIP test done otherwise you will start playing an expensive and time-consuming game of 'poke and hope'. It could be the beginnings of the dephaser pulley. Hopefully not as this will set you back a good £500. A failed DP should show a code on the CLIP test. Renault Build A Better C...
I generally have a low opinion of after-market warranties. If you go for one then check the small print very carefully for exactly what is and what isn't covered.
Thanks for the comments. I shall be in France for Easter so I shall go into a Renault garage and try and get a new knob. Poor design though - you wonder how much development time is spent by manufaturers. My knob was perfect until a mechanic took the car for a test drive... 30 years of Renault owner...