Right, after major hassle of getting my wardrobe and bed (plus desk) out of my room and removing my carpet, I have managed to take some photos of the problem (no digicam available so got some poor quality ones with my nokia6230)
This is what i found - generally the boards are flat but the problem areas are unfortunatly exactly over the path into the room and directly where my feet are when i sit on the PC so I really notice them - in the past I screwed some screws in to stop creaking which worked (i will cover the whole room, as last tiem i couldnt be arsed to take everything out...) which you can also see...
Anyway:
This is a general layout of the floor.
Now rather than hardboarding it I think its easier and quicker to fill the low one with a sheet of hardboard and sand it - and for the one that is higher than the rest it should be ok to just sand it down a little - and for good measure screw in every board to stop creaking.
Any other options?
In terms of GAPS between boards - the biggest being this one
I am feeling this sander- should be OK?