I've had a bit more fiddle today and seem to have cured it. There are two multi plugs going into the back of the warning light cluster. If you take one of them out the brake warning light comes on. I've fiddled with it and sprayed electrical cleaner on the pins and now after a brisk drive round Leicester with lots of hard braking in the rush hour traffic I didn't get a flicker at all.
I've also had a fiddle with the front hubs and suspension. The bearings had settled a bit after having renewed them just before Christmas, so one of them needed the nut taking up about 20 degrees. I've also tightened the bottom shock absorber mounts because the rubbers had squashed a little and they weren't as solid as when I first fitted them. All nice and tight now. Swivels still seem to be spot on and didn't need any shims taking out.
I've also had a fiddle with the front hubs and suspension. The bearings had settled a bit after having renewed them just before Christmas, so one of them needed the nut taking up about 20 degrees. I've also tightened the bottom shock absorber mounts because the rubbers had squashed a little and they weren't as solid as when I first fitted them. All nice and tight now. Swivels still seem to be spot on and didn't need any shims taking out.