A7 OOH, It now looks to me as if you have got a later wiring loom together with an early master cylinder. My car also does not have a covered pedal box and according to the Tony Weale book the covered box was introduced in 1986 so according to that yours should have one! Are you sure the car was supplied as a CKD, i.e. all parts supplied new by Caterham (that can be checked from the chassis no.). Mine wasn't, I sourced the drive train and some of the suspension myself. You say the brake test switch works - all you are doing there is testing the bulb, that is what the switch is for. Brake failure or low fluid level will light the bulb without operating the switch.