The Shiver uses a dual-filament lamp with each globe running a 12V and 21V rail.
12V for tail-light, 21V for brake.
Turns out one lamp was blown on both rails, no tail-light, no brake light.
So - time to replace them (both). Yeah, one is still good, but if I'm going in there to replace the blown lamp I might as well freshen them both up eh?
Seat comes off and 8 fixings are removed:
2 x 12mm hex bolt
2 x 10mm hex bolt
2 x Phillips screw
2 x 3mm allen bolt
Move the grab-rail assembly and main fuses out of the way to allow access to the tail-light reflector and lens