Well, the dash lights problem turned out to simply be an old dodgy light switch, so I cleaned up the contacts using contact cleaner and compressed air, and that was that. Sorted.
As for the oil temp gauge reading maximum as soon as I turned on the ignition.. Well, this is embarrassing, but the car has a dipstick temp sensor, and I’d obviously disconnected and reconnected the feed and earth the wrong way round on the dipstick! Problem solved. :-)