It's not really a plothole so to speak, but attempting to bash out a timeline for The Dark Is Rising Sequence requires ignoring decimalisation issues -- specifically the fact that the Drews can have farthings as pocket change in the first book (which would date it to some time before 1961 or 1962, when the farthing was taken out of official circulation) and yet The Dark Is Rising, supposedly set about a half-year later, is clearly post-Decimal Day...a gap of about a decade.