The sale lasts for the rest of the year.
From the sales sheet:
It's that time of year again, and once more we're running our Christmas sale. As in previous years, it’s a straightforward 15% discount off everything that we make – no minimum order, just 15% off everything (with one or two exceptions - full details are on our website). The sale runs for the while of December, ending at midnight GMT on December 31st.