Fiat Punto will not start, I turn the key and just hear a clicking sound

My Fiat Punto sometimes will not start, when I turn the key I just hear a clicking sound. All the dashboard lights come on and the central locking works
fine. Usually if I wait 5 minutes it will then start and then I don't have any further problems. This doesn't happen all the time mainly when I'm running late! The car has been to the garage where they did lots of different diagnostic tests and could not find anything wrong. Has anyone got any suggestions as to what could be causing this problem and how it can be fixed please?

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