Hi again Suzy. I will be honest i'm quite puzzled and without actually seeing the car it is very hard to work anything out. Hopefully somebody will be able to read the information you have told and give a more definate cause.
It's possible that the garage who first fitted the belt have somehow fitted it incorrectly. Normally when you fit a belt you turn it over by hand to check everything is free and the engine isn't locking so as to make sure it is timed correctly before starting it. If they have fitted the belt a tooth out from the corrent position it may have turned over and damaged the valve though i'm not sure unless it actually fired up that the starter motor alone would have enough force to do this normally the engine would just lock up. The damaged valve would explain the rough running though.
Though incorrect fitting of the belt could cause extra stress on it leading to early failure I would say you have just been
unlucky with it going so close to the time it should have been changed.
From what you say there does not sound anything wrong with the pressure in cylinder one though the white coloured plug can indicate that it is not getting enough fuel. This could possibly be down to a blocked fuel injector on that cylinder but that alone would not be enough to stop the car from starting. Since it also is not showing up any fault codes it may well be worth checking the
particulary number 1 as these can lead to the car not running right and also starting problems.
When you try and start it does it make any attempt to fire up?
Like I say i'm sorry I can't be of more help it's very hard without being able to see the car though maybe somebody else will pick up on something.
If you don't mind me asking where are you located I use a few Renault forums and know of a few very knowledgable speciallists around the country.