Yes. Turns out it was pretty simple once focused on it. Between removing both grounds and reattaching and then tracing the contact from the brake light element back to the socket that connects the wires to the membrane I discovered the contact for the brake light element was slightly corroded - hard to see but once it was sanded/cleaned everything was fine. Took about 10 minutes in all.

Glad it wasn't more fundemental wiring problems - which I've had previously - trying to find a short that caused a fuse to blow every time the brake pedal was pressed took quite a while to find. If Volvo's had better wiring the human race would have been saved millions of hours of diagnosis and fixing...