From the back of the book Time Zero

Justin Richards has no cat.  He might (or might not) have had a cat when he was a child, but if he did he never ever put it in a box.  He does have two children, but that's not the same thing at all.  Believe me, if you have a cat and you're thinking of trading up for kids, there is a big difference.

When he isn't busy with his children, Justin acts as Creative Consultant to BBC Worldwide's various Doctor Who book ranges as well as doing some writing of his own - novels, audios, television, non fiction and other 'stuff'.  Presumably this is done during the time he would have had to spend with the cat, so given that Justin never has enough time for writing, he's thinking of not getting another one.