|I fear that this may be cause for some Occupational Health & Safety concerns.|
The existing boards have actually done pretty well, considering that they were laid on bearers (jarrah, I think — some sort of very dark red-brown wood) straight on to the clay almost a hundred years ago. There's only about 100 mm free space between the floorboards and the dirt.
I discovered a little brick step that I didn't know existed. I guess it's a rudimentary moisture barrier or something, but I don't know for sure.
The exposed bearers seem solid enough when I whack them on top, but the bottom half, in contact with the soil, is basically just wood-pulp, so they will have to go.
I bought a 2440 x 1220 mm sheet of 18 mm strand-board to lay down as a new floor, but having gone this far I'm actually wondering about ripping up all the boards, a quarter of the shed at a time, and laying the floor in concrete.
It's not exactly very level, but it's firm and dry underfoot. It's also about 25-30 mm lower than the original floor level, which makes my work surfaces relatively that much higher.
I shall try it out for a while just as it is, and if it's not too obnoxious, that means I can use the strand-board elsewhere in there. There's plenty of floor that needs it; if anything, it gets worse the further back you go.