I'll give that a go Torstan. Strangely, even though I use masks routinely in terrain software, I've never really used them in Photoshop.
There's so much in Photoshop that I've never used.

Just been fiddling with that for 30 minutes and I'm not getting anything.
Here's what I do: New adjustmentlayer > brightness/contrast
It does not give me the option of filling the mask with black, just white and various colours. so I choose white.
Then a dialog pops up so I set the contrast right up to about 80%.
Then I fill the layer with black and the mask goes black as it should.
Then I choose white paint and paint on the top mask layer and nothing.

Should I be creating a duplicate of the layer first?
Should I be selecting "use previous layer to create clipping mask" when creating the adjustment layer?

What's currently happening is if I keep the mask fill as white and paint with black, it lowers the contrast back to the original layer beneath
-that's what it should do, but if I fill the mask with black and then paint with white, I get no change when I paint so at the moment I can lower the reduce but not increase it.
I'm not sure about the brush opacity step you mentioned. I don't see any way of altering that in the Brushes Window.
I'm using CS2 here.

monks