why oh why wont my megane start?

Only had my renault megane cabriolet a few months nothing but starting problems, wet or cold never wants to start, when i first had the car felt sluggish took to local garage got them to check it over said all was ok, week later a knocking sound started couldnt find where knocking was comin from took back to garage, the bloke scratched his head called other bloke then both scratched their heads kept car in overnight, found out it was the driveshaft knocking bloke said it was all corroded couldnt understand why as car omly four years old, got that fixed at the cost of £180, a few weeks ago it died outside friends house AA bloke jump started checked battery and alternator said all was ok couldnt understand why it had died, yesterday morning car dead no life really p****d off with this car going to try changing 15A fuse (#19) and TDC sensor then geting rid of car