Slack Hill (or more correctly Amber Lane) was worse before it was re-aligned in the mid 70s (there's some shots from back then on Old Chesterfield Pics on Facebook.) That was the first car accidint I ever saw back in the 60s and my dad, being in the St Johns, got out to assist.
I also remember my moped wouldn't go up without pedal assistance and a hear later my (nameless) friend clocking up 100 on the straightened road so it narrows the change down to '73/'74