Hi Alasdair - re the twenty year curse I had never heard of it either, this is copied from Wikipedia - The term Curse of Tippecanoe (also known as Tecumseh's curse, the presidential curse, zero-year curse, the twenty-year curse, or the twenty-year presidential jinx) is sometimes used to describe the pattern where from 1840 to 1960 each American President who had won election in a year ending in zero (such as 1880 or 1900) died in office. The pattern ended with Ronald Reagan, who won the presidential election in 1980, but did not subsequently die in office.
I agree with shorthorn's post all you need for this crossword is Wikipedia!!