OK, thanks. I'll look for the hump.
The bottle has four segments and the fourth segment from the front of the car is where the cap is. When you look through the cap, this segment is seperated from the third segment with a wall containing a hole.
Presently the level is just up to the hole in the wall, so no fluid in the fourth segment under the cap. I guess this means that the hump will be dry (but I'll check).
I haven't been too worried about the level because it doesn't move, and because it appears to be higher than the radiator (so, I suppose the radiator is full).
If the radiator is full, and there is plenty in the bottle (albeit not covering the hump), I couldn't see why the level should be higher.