This morning the screed upstairs did not look too good. The screeder said it was the quality of the sand and that it was level. I discovered that there is a British Standard in screeding. Apparently over a 2m straight edge there can be a tolerance of +/- 7mm (called SR3). A flowing screed can achieve a slightly higher tolerance apparently. Anyway, I called a halt to the downstairs screeding while I decide what to do.
Meanwhile, the screed upstairs is drying, giving off heat on one of the hottest days of the year.