The car will not start, battery seems fine

Hi There. I have been having a problem which has been occurring now for about 6 months. On occasion when I try to start the car it will not start. The battery does not go flat so it seems to be some kind of electronic issue. It has probably happened now around 6 times and I never know what to expect when I go to my car. It makes it very unreliable. Once I managed to get a push start, which meant I was able to trundle the car to a nearby garage. When I did get it push started above, there was absolutely no power. Every other time I have been unable to get it moving even on a hill start. The strange thing is that if I put clutch down and handbrake off and start in 1st gear with foot on the revs, the car attempts to move forward, but only does this in inches. I have had a friend check the diagnostics and no codes have shown up as being a problem. Please help me.