In reply to girlymonkey:
Don't worry too much about the lack of foundations, quite a few old places are like that. Just be careful about anything that introduces loads of water.... busted drain, blocked gully, overflowing rainwater pipe etc.
As for the rat, they like eating expanded foam (learnt from that mistake myself) and will gnaw through almost anything, even cement and sand given enough time.
Use conc block or something like that and bed in as thin a cement sand as you can make it, ie get the block to be a tight fit.