Radio Luxemburg for me as well. Also Mike Raven's Sunday night show on the BBC and Emperor Rosko on Saturday morning who used to slip in a few soul records that weren't in the charts.
Mark, yes some were discovered in the seventies as you say, but maybe many more weren't in the hands of DJs (or those that sold to them) or were still to be discovered from the US.
Julian B's post of 5th December in the Bostock's in Bradford Market thread is a good example of how records were found but didn't suit the type of records that were popular in clubs back then but got played in the eighties .
A few I remember having in the early seventies that only got played later include What Can I Do- George Kirby , Can You Remember- Rhonda Davis, Special Kind of Woman - Paul Thompson (all sold for peanuts).
However, I think some records were just never found until the 80s or 90s. No one who found Jackie Day - Naughty Boy in the early 70s wouldn't have bought it blind and then not realised it's potential. Also the acetates from US studios didn't start getting liberated until later on, those who found these could probably give more details.
Rick Cooper's post in Early Radio (60's Onward) What Were YOU Tuned Into? was marked as the answer