I think someone's put on here somewhere that even he realises they are worthy of a spin off but he just hasn't got time. He did hint they may return though.
I thought they were brilliant - so well realised in such a short amount of screen time. But then the idea behind them is so great - a Silurian detective hunting down Jack the Ripper & other villains in Victorian London with her trusty human sidekick & lover. Fab! They totally deserve to come back somehow.
Someone has to make it happen!