Non starter after a run - Citreon C3?

My Citreon C3 is both automatic/manual by paddles and is petrol. After a 25 mile run if I stop and start the engine a few times it won't start on the third/fourth time. I thought it was overheating and garage put a new radiator on. same day it did it again. For the past few week I've carried a battery pack because it will normally start with that. So I had the battery and alternator checked at a garage and was told they were good. Tonight I did 35 miles stopped at the crossing and heard two ping sounds. Then the automatic went and I couldn't change gear not even by the paddles in manual but the engine was still going. I turned engine off and then back on. It wouldn't start but automatic light came back on. I left it for 40 mins. Went back and it started and the automatic was working. Does anyone know what this could be please? Thanks