I've just figured it out by reading some other posts.. for those who are curious though, this is what I did; created a slab the same shape as the basement, and set the bottom of the slab to be equal to the bottom of the basement. I se the height of the slab to an arbitrary amount, high enough so that it was sticking entirely out of the ground, then I used the element info pallete to find the volume, wrote this down, then used SEO to subtract the mesh from the slab, got the new volume of the slab, found the difference between that and the first, and voi la!
hopefully someone other than me finds that info useful
cheers,
dan