Think most people would agree that the "golden era" for the discovery of the classic northern soul tunes (now sometimes referred to as "played out oldies" in certain circles) was 1971-1974/75. This obviously encompasses Torch, Mecca, Va-Va, Catacombs & Wigan.
Wigan did not open until 1973 & was heavily influenced by the other clubs listed above. In 1976 the "oldies" nights overtook the regular Saturday nights in popularity.
In fairness I think Wigan clawed some credibility back in 79/80 when Richard Searling was playing some awesome tuneage.