Bah, just been doing some testing on the dash and have discovered a wee mistake. 
I have wired in a timer that tests the brake fail buzzer / light when the keyswitch is made. It lights the "brake fail" light and sounds a buzzer for 1 second. This all works perfectly (I even managed to get all the diodes not only in the right place, but also in the correct orientation)
Now, the more astute will realise that the brake fail circuit also needs to be activated by the switches on top of each brake reservoir. What is installed on the dash is only a test switch loop to prove the circuit works.
I, not thinking again obviously, have neglected to attach any wiring to the actual brake fail circuit. The test circuit works fine but there's nothing to connect it to the actual brake reservoirs. That is now the job for tomorrow.
I blame the solder fumes :-)