Renault Megane Scenic R Registration

Hi, I have a megane scenic R reg 2litre petrol car. I am having a problem with ALL the electrics. Some mornings I go to the car and everything is dead. I can't even unlock the car. Or, I can get in the car, drive it normally, get out go to lock it and the electrics have all died again. Some mornings, when I get in and start it, I can tell the electrics have switched off overnight because the radio needs re coding and the digital mileometer has reset to 0. It is an intermittant problem but it means my wife can't rely on the car. It has happened again today. She's driven to work fine, got out, gone to lock it and all the electrics have died so she cannot even lock it. Ny advice woyld be appreciated. Its as thought the battery leads are loose but I have checked these and they are all very tight. IT'S DONE 106000 miles.