Reporting back after my visit to the dealer today. They judged that the symptoms suggested a possible problem with the starter or the solenoid. Neither of these would show up on a diagnostic test as they are not part of the computerised monitoring system. They did, quite rightly, point out that intermittant faults like this are the most difficult to solve.
However, they did spend time checking and improving the electrical connections to the starter/solenoid and also the engine earth connections. They offered to take further action if the fault could be seen first hand by them.
Not a complete result, and I fear I will worry now every time I turn the key.