1999 Vauxhall Corsa Intermittent non-starting problems

I have an intermittent starting fault, the car will start first thing in the morning but occasionally it will not start later on in the day. I have tried foot on floor for possible flooding and have waited for a while before trying again when sometimes it would start. On Sunday this did not work so the AA was called, guess what he turned the key and it started ! The AA man said that he could not diagnose the fault unless the car would not start but suggested it may be the "crankshaft sensor". Does anyone else have any suggestions and how do you find the fault if the car is running? Do I have to wait until it breaks down again and then get it towed to the garage. I would have thought that this was a common problem as I remember it happening to my Ford Orion years ago. Any help really appreciated, many thanks, Kenny.