My Peugeot 206 diesel sometimes won't start?

I guess it must be an electrical problem.Have replaced the starter motor. Also replaced the pre-heat control unit. You get in - the motor turns but doesn't start. Leave it for an hour, or maybe 2 days, and it starts first time.The little sqiggly sign sometimes doesn't come on. Any ideas? Also there is a spanner with "900" next to it comes on the digital display. Anybody tell me what this is? Never had a car like this. Give me the good old days when you could mend things - not all of this electronic crap designed to cost you money. Anybody agree? Appreciate any advice. Mick