I had a recent experience with a sometimes starting car. To sniff out what it was I plugged in my battery tender ( this one -> https://www.amazon.com/gp/product/B0051D3MP6/ ) and that fixed it. Yes a little annoying to unplug and remember to plug in when I got home but it did lead me to a failing but not failed battery.
Be sure to start with a good old battery terminal cleaning first.
My Peugeot 206 HDi picks and chooses whether it will start first time or not. On the occasions when it doesn't start I will get in the car try turning the key and the electrics will all turn off, I get no ignition response so I have to open the drivers door to start the electrics and then try starting the car again. Sometimes I have to repeat this several times until the electrics stay on and the car starts. Has anyone got any idea what the problem could be?

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