Well, that's it. The dash is finished (and the "snakes wedding" that is hanging out of the bottom of the dash is merely the hen night prior to the connection of all that wiring to the car; now that will be a snakes wedding of epic proportions. After this, I will spirawrap the remaining loose wiring.

The brake fail circuit (or lack of) episode was drawn to a successful conclusion. It now all works! The brake fail light comes on even if the buzzer is silenced (I think I did intend to wire it like that...)
I do have reservations about the speedometer (in that it never worked properly in the car's original incarnation - no pun intended) so this means I have been looking at Arduino powered GPS speedometers... however the KISS ethos shouts an emphatic"NO!"

but it really means "not yet" :-)