Ren Hoek wrote:
Having spent the whole weekend laying a 3mm hardboard floor covering,
some of the boards have starting warping and lifting slightly.

What haven't I done properly? The boards are all screwed around the
perimeter every 5 inches or so? The floor underneath is concrete.
Somebody said I should have perhaps dampened the boards before laying,
is this correct?

It'll help.
The problem is it's bloody hard to tension it properly while being laid.
If you don't, it'll bend and flap.
Dampening makes the boards swell a bit so that they are wider and longer,
so that when they shrink, they are under tension.

Several ways.

The right way is to lift it, dampen it, and refit.
Alternatively, simply screw into the middle of the bump.
Repeat until flat.