My renault is shuddering and juddering and lacking power. Anybody got any ideas what it is

You might want to go out and look around the engine for vacuum leaks, that would be the most likely cause.

Most common cause of rough running Renaults is a faulty ignition coil/s. Theses are found on most of the 16 valve engines and are the 'pencil' type coil. As your engine management light has illuminated there SHOULD be a stored fault in the ECU memory, most garages will have equipment to read the fault codes, however I have had Renaults that although the engine is clearly mis-firing the diagnostic tool has given the ECU a clear bill of health. Most motor factors will stock the coils so you do not need to pay extortionate main dealer prices.

Thank you for help. It was the ignition coil. Now running like a dream .