It just occured to me that..

..I never wrote anymore about the drain. My housemate's dad sorted us out with some chicken wire to cover the drain up outside, and we've got a plughole cover inside as well. Although it doesn't always seem to want to stay in position, so we may be investigating other designs to see if we can find one that fits better. Our neighbours have also shot a couple of rats (not entirely sure what with, but we didn't feel like enquiring too deeply into that one), so hopefully that might help with the problem!