HC -
I think it used to be 0.5 miles in the late 70s but was never 0.25
AFAIK.
Seem to remember that when I started in Aberdeen in 1986 it was 0.45nm (don't know why it was not 0.5, but pretty sure it was 0.45), and that was with the old monochrome radars that had a much poorer definition than the newer ones.