At the boys school this morning, chilling while waiting, and listening to Motorhead and Monster Magnet. Turn the key, and nothing, nada zilch.

Not even a click from the starter

Hammer out, smack the starter, no joy.

Call a friend, on his way, then a roll back down the hill in reverse, and it fires up! Turn off, restarts on the key.

Cancel friend, drive home, now all good and starting well!

I have checked the exciter wire - it seemed a little slack on the connector, but the starter looks new, but cheap..... no identifying marks...

Time will tell if it needs replacing, but I have been having symptoms where I turn the key, and there is a half second pause before anything happens....

Looks like you have found the problem. The connector to starter solenoid is a common problem.

I had similar on my 300tdi, turned out to be a loose connector to the solenoid (is this what you call the exciter).

Thanks for the reassurance - after a quick squash of the connector with pliers, it does seem a little more eager to start - time will tell!

Mine would start fine with normal running, but if I had been towing on long journeys (engine working hard for some time) the starter would be dead, nothing at all when turning the key until it cooled down a bit.
Once I replaced the connector it's been fine for almost a year.