Hi Ben,
Things to look out for:
Lesser Scaup is smaller than (Greater) Scaup, about the same size as a Tuftie
The head shape is different: Scaup is smoothly rounded, Lesser is peaked, with almost a short stubby crest
The drakes, Scaup has very finely vermiculated black lines across the whitish back, on Lesser they are coarser.
In flight, Scaup has a white wingbar running the length of the wing, secondaries and primaries; in lesser, the wingbar is only white on the secondaries (inner wing), dull grey on the primaries.
Next job . . . telling Lesser Scaup from a hybrid Tufted x Pochard . . . now that's really difficult, but these hybrids happen, and have to be taken into account. Well, tomorrow, its too late to do that now!!
Michael