Dave, it could be your ecu in that case....might have completely died. you can pick up second hand ones for £25 or so...
what you need to do is.....take the starter mototr out (2 bolts, very easy), then try and turn the oter over as if you were starting the car, if the motor turns, then its ok. if not, use your volt meter to check if any powers getting to the stater motor when the key is turned.
im not sure if renault put there immobalisers in the ecu (alot of cars do)....the light for the immobaliser should flash every second when the ingnition is switched off and the key is removed.
if it is a fault with the ecu, it might just be the wiring loom, they arnt very well made, and after time the wires rub thru, check the loom for splits.
chack to see if your head lamps come on while the ignition is on....if you hve a dead short, this would stop it from working. saying that tho, you've tested the volts off the bat while trying to start and said it was ok...
let us know if any of this works....iv got more to try if it dont.