Water water everywhere (that it isn’t supposed to be)

Well isn’t that just the rub? You have a builder not coming until the weekend, and the first most savage of winter storms makes the back chimney leak. Each day now I am checking the slightly brown patch as its very slow march from the top corner of the wall in the back bedroom continues. The builder reckons that it’s a defect with the lead flashing. I really don’t care what it is as long as it gets fixed quickly and before any real damage is caused.