For the floor I opted to use 50mm Celotex PIR board. This should provide a decent level of insulation underfoot.
I started by lining the floor (Which was basically the HawkLok shed base, and the floor rafters) in a damp-proof sheet. Basically this is just a very thick black plastic sheet. It covered the floor completely, and will stop any water from under the shed damaging the insulation.
The Celotex was laid between the rafters. To cut the Celotex up I used a standard saw, however a kitchen knife or knife with a vert fine set of teeth will be better, and leave much less dust - never, ever cut Celotex up without an asbestos-grade face mask. It's not in any way cancerous, but it will cause breathing problems for a couple of days, and from my research it turns out that the cheap paper masks dont actually do bugger all, so make sure its asbestos grade (they are around £5 each).
Once the insulation was fitted, any gaps were filled with expanding foam. I didn't need to worry about using foil tape around the edges as the celotex was on the sealed plastic sheet, so no air or liquid restriction was needed.
The floorboards were then laid from front to back, they interlock a bit like laminate flooring but are made of solid wood. They are then nailed down to the rafters.
I was actually pleasantly surprised at how stable the floor was. I expected a bit of 'bounce' to it, but it was pretty rock solid.
Once the floor was done, it was time to work on the ceiling, which I'll cover in the next post!