Well, on a lighter note, since we - non-feline folk - can't decide it, shan't we call in a real cat to tell us what's what? At least a cat would be able to tell where his mouse is buttered -- I beg your pardon, how his MICE are plural(ised)!

Please how do you pluralise a phrase like 'head of department'? Is it 'heads of department' or heads of departments'?