I've found a small circular off/on switch on the driver side centre console cover panel, more or less where my left knee is. It's pretty well central in the photo.

Iirc that's is the transport switch which disables a number of things (active suspension etc) so it can be loaded on a transporter from the factory for delivery to dealers etc.

Thanks, Dolphy. I shall leave it totally alone.

No, that's not the transport switch. It's not even original so it would need investigating to see what it's connected to.

Gentlemen, thanks very much for efforts. I traced the wiring to a baffling set of thousands of other leads, but I suspected it was the telephone hands-free system.
Then I noticed the "Nokia" on the switch itself. Very small, but there it was.