Around the late 60s is probably when they atarted using pre-recorded announcements on a regular basis. There were probably staff on the relevant platforms shouting it on their own initiative from time to time.

Gee I always thought that Mind the Gap referred to kissing a English gal!

It's a wonder you weren't locked up, PQ.

http://en.wikipedia.org/wiki/Mind_the_gap

Interesting discussion of origins of Mind the Gap (flanneruk of course was correct) - at the same time of the original 1969 recording, still used at some stations on some lines, the phrase "stand clear of the doors please" was also used.
It was decided to use a recording after the practice of conductors and drivers yelling warnings about the gap.