What's wrong with my 54 plate Corsa not starting?

We have a 1.0 Corsa Life and once in awhile it refuses to start and just turns over without even attempting to fire. Leave it overnight and it starts fine! It seems to happen when the car's been left for between one and four hours. The crankshaft sensor has been changed to no avail. Nothing shows up on the diagnostics when plugged in. Can anyone suggest anything? The AA man said something about lambda sensor! What's that?