Steven Campbell wrote:
I've cut out about 1 metre square of floorboards at my front door after
water had been getting in and had rotted the original floor boards.
I've replaced them with new ones which are a tight fit!

Should I cut off a couple of mm to allow for expansion?

Presumably you've cured the water ingress problem? It's moisture in the
wood that will cause it to expand, so assuming a normal warm dry house,
the boards are more likely to contract than expand. So, no.

Is it ok to screw these boards in as apposed to nail in?

Yes - opinions vary but IMHO it's usually preferable anyway because (a)
they are less likely to loosen off with time and start creaking
(especially in a high-traffic area like this) and (b) screws can easily
be undone to lift the boards if you ever need to.

David