Update; problem appears to have been solved. I decided to check out the micro-switch on the luggage compartment catch/lock, as the compartment light had stopped illuminating when the hood was open. The amount of crud in there was unbelievable. Anyway, cleaned it all up, reassembled, and the light now works again, and hey-presto, my dash LCD's switch off when the car is locked and armed. I have no explanation for this, and I'm keeping my fingers crossed that this was the cause all along. Time will tell.
Next project is to take the cylinder heads off my Stag to cure an annoying coolant leak, and convert the engine to EFI
Next project is to take the cylinder heads off my Stag to cure an annoying coolant leak, and convert the engine to EFI