[CharlesJones]

I had the exact same symptons - a sporadic no crank, which just got more frequent until I got stranded at work with no crank at all. When the fault was sporadic I would sometimes have a no crank situation, but then if I left the car for 20 minutes and then try again, it would crank fine! And every time it wouldn't crank the car would always bump start fine of course.

To find the find the fault I did a voltage drop test as others have suggested. I soldered two crocodile clips to long lengths of wire so I could have the multimeter next to me on the passenger seat whilst I turned the ignition key - that way I could test for voltage drop across the starter whilst still sitting in the car.

In my case I found the voltage drop (and hence the fault) across the cable connector to the battery terminal. The terminal and connector looked absolutely fine, but meauring between the connector (the bit which connects cable to battery terminal) and cable I found a voltage drop. I simply snipped the connector off and replaced with a new one (which cost about 5 euro) and my problem was completely fixed!