I had a problem today - car wouldn't start (1.5 DCi on a 54 plate) - all the electrics working, no warning lights, but when I hit "start" - nothing happened. Battery seemed fine, headlights Ok, etc, etc.
Hunted the web and the forum but no exact match, closest was a suggestion to check the starter solenoid

On doing so I discovered that the terminal of the earth cable had snapped in two - obviously had been stressed/bending and just failed (looks like copper).
So I went to Mill auto, bought an earth strap for £12.50 - had to transfer the megane battery terminal to the new strap but otherwise painless - fitted it and started first time

Would have cost me £60 for a tow to our nearest renault dealer, probably £50 for diagnostics and who knows what they would have come up with before finding the earth problem.
I'm happy and maybe it'll help you too?
Cheers.