Intermittent starting problem

[DaBigGinger]

I've got a funny problem with the G. 9 times of of 10, it will start without issue. However, on occasion, it won't turn over. I turn the key, I can hear a single click which I presume is the starter solenoid and I can hear the fuel pump, but nothing else happens. If I try again a few times, sometimes it will decide to start and other times, nothing! If I push start it, it will start without issue,

My first thought was a sticking starter motor, so I've had that replaced, but the same problem still happens. Any ideas anybody?

Thank