i had the same prob more or less on my old 306, the drain plug underneath had become blocked with leaves (do you park underneath trees?) so it just filled like a little pool, check to see if theres any drain points underneath your white box
was it raining last time it wouldnt start?
does sound like an elec prob tho, do you have a spare key that you can try next time it doesnt start so you can rule out the immobilser?