I've had more pressing problems to rectify since getting my Defender 90 a few months ago, but here is a more minor irritating problem.
Usually occurs when going downhill, the battery & brake warning lights come on.
I guess it's a bad earth or cable shorting to the chassis somewhere, I've checked the battery connections and will need to check the wiring generally.
Thought I'd post this just in case someone knows of a known problem I should look for...

It's not only when I'm braking either...

I had something similar to this, when braking going downhill. I pulled the multiplugs of the back of the warning light panel and sprayed the pins and sockets with electrical contact cleaner and it was much better. However the brake light continued to flicker. I replaced the switch on the handbrake to no avail. Eventually I solved it by means of a modification to the brake fluid level detector in the cap of the brake fluid reservoir. There's a float in a cylindrical chamber. Although there was plenty of brake fluid the float switch seemed to be causing trouble. I reamed out the tiny hole in the cylinder, so the level of the fluid in the float chamber can equalise with the fluid level in the rest of the reservoir more quickly. Seems to have done the job and my flicker is gone.

Ah, cool, I'll look into that, could be the problem, exactly why I posted, thanks Brown!

I decided to try topping up the brake fluid as it was a little low, not below the minimum though.
Topped it up to max.
Also a brake light was out so checked it.
Found it to be at a funny angle, not seated correctly and the bulb holder was melted where it was resting.
Not possible to seat it right, need a new bulb holder.
The bulb was OK so left it out.
What ever, battery and brake light not flashing any more...

Will replace the bulb holder as soon as possible.
I guess the bulb holder was the problem rather than the brake fluid level.