How can I "seal up" cracks in my wooden shed between wooden slats/floorboards?
Just lay some tongue-and-groove OSB plywood on top of the planks.

For a 6x8 shed, you should only need two 4x8 sheets - which should certainly cost a lot less than the £120 you would pay for sealer.

Or for that matter, just have them use T&amp;G plywood for the flooring in the first place.