Clare,
If you are just using normal water then it could just boil away gradually. Best thing to do would be to flush the cooling system and fill up with the Premix coolant that Renault sell. Is about £12 for 5 litres (I think the £6 one is for a 2 litre bottle or something).
You could also have a slight leak, maybe a loost or hairline split in one of the hoses. Have a look round the engine bay for signs of water stains on parts of the engine bay, that will give away the location if its a leak. Also, remove your oil filler cap and look inside the cap to see what colour the oil is thats around the top. If it looks like mayonasse (sp!) then you either have a leaking head gasket or might also mean you only do short journeys. You can double check this by taking out the dip-stick and checking again for the mayonasse coloured oil. If its normal oil colour, all is normal.
Could also just be an airlock in the system, but this is genreally caused by an air leak somewhere, but could be a very tiny leak.!
But to start with just top up and see what happens. Id advise a complete flush and new coolant tho.