st. martin in the fields, london

this drain can be found next to the doorway to the royal pew, but as there are conventional lead downpipes for rainwater from the roof, what was this for? my first thought was sewage, but a close stool would have been more likely at the time. I have so far found no trace on internal plans or elevations, but these don't cover the upper floors in any great detail anyway. built by gibbs 1721-6