Baldwins Herbalist in Walworth Road.....large one is £13.49 according to website. I've had my one years, it's the real mckoy., ceramic with wooden handle.
check to see if they have one in stock, the number in 0207-703 5550.
Selfridges has a good range (glass, olive wood, ceramic etc.) and they also stock the carved stone P&M that Jamie Oliver uses. Whilst you are in the neighbourhood its worth checking the dedicated kitchen shop Divertimenti on Wigmore street as well.