To answer the question, modern always was a feature of the northern scene throughout the 70s. After the demis of the Mecca / Cleethorpes, Ronnie Love, Eddie Holman, Silvetti, 7th Wonder, Lovelites, Flaming Emeralds etc. Indeed Lou Pride 6 years old when it was first played as were many northern 60s records. That's the equivelent of playing a 2008 record now.
But the first real main "tranche" of modern after the Mecca demise, came in with a vengance in 79, when John Anderson sold Richard Searling a number of sounds that went on in time to become classic oldies: Roy Dawson, Pages, Court Davis, Sammy Gaha, ZZ& Co, Ike Strong, Alfie Davison, Bobby Thurston etc. The feeling at the time was that the 60s newies had all but dried up and some new music was needed (whether you agree or don't agree - that is what happened). Suddenly DJ sets of people like Richard, Poke, Jonathan, Pat Brady etc. were full of these new discoveries mostly covered up of course. Soul Sam and others were also in there, he traded "Cheating kind" for Richard's Larry Houston!
It seemed to happen almost overnight.