I remamber being really scared by a mid 60's episode of "The Avengers" where a gigantic bird of prey (it actually turned out to be a mad scientist disguised as one, but I didn't know that at the time) scaled the walls of buildings, smashed windows and clawed various unsuspecting victims to death. It gave me nightmares for days !
At around the same time, I also used to find an american sci-fi series called "Lost in space" very scary at times. Unless I'm imaging it, I seem to remember that one of the original "Star Trek" episodes was deemed so disturbing, it was never repeated (but I may be mixing it up with something else).
Crispybug - I think you mean a drama called "Threads" that was screened around about the mid-80's. It was about the after-effects of a nuclear bomb on a large UK city (Sheffield I think) and was actually very,very good indeed.