Ok, just checked all the necessary fuses, bulbs and lights and everything seems to be working perfectly well.
Also noticed that the light only comes on under 2 circumstances:-
1. Switch on side lights AND press the brake pedal.
2. Switch on headlights.
I cant understand why it comes on with the headlights given its the REAR lights its ment to be indicating as out.
Can something else in the braking system be causing this aswell? Pads, discs, brake fluid. Maybe being stupid but the obvious has been checked out and is in order.
The same thing happened to me a few months ago, strange as it may sound but the problem is with the bulb holder in one of the rear lights on mine, could the same problem your having.
On the Aristo all the back lights act as a brake light as well as side lights, on the UK GS, only the outside pair act as a brake/side light, so you may be looking at the outside pair only - the bulbs that have a twin element require a better earthing point, if the bulb holder has rust in the connection, the earth resistance is lower thus bringing the warning light on the dash.
I needed to clean and remove all the rust before the warning light went away when I braked with the side lights on.
Hope this helps
BR
Paul