When the start relay developed an intermittent fault it behaved exactly like Karlosfandangos M3W. More often it is the relay contacts that are dirty. I took the relay apart and cleaned the contacts and that solved the problem.
Engine now turns over continuously for as long as you press the starter button where as before it would stop after a few turns and you would then have to repress the start button to turn the engine over again. The engine starts eventually but the symptoms are very similar to the battery not having enough charge.
