Hi Graham,
I am relatively new to the site also and as although I have been enjoying soul music over half of my life I am still only 33 and just when I feel I have scratched the surface of the general sound of what floats my boat something comes along and knocks me off my feet.
I, like you missed the boat for the original venues (such as the Wheel, Casino et al), in fact that ship had sailed long before the twinkle in my fathers eye! So I do understand the journey you are on discovering what is known to many as the 'scene'.
In my early days I thought I knew what 'Northern Soul' was, most likely the said '150 tunes' you refer to were in that list. My first visits to soul do's, all dayers & weekenders I was left baffled as 'where are my 150 dancers?! I don't know any of these records?!' - what changed was my attitude, I stop trying to recognise the track but simply enjoy it, I stopped trying to analyse everything I heard but paid attention to those records that reached out to me.
Over years of searching and listening and digging and searching and listening and digging... I have found that I will never find or even hear every 'northern soul' song, I find I don't even like some tracks that my fellow soulies would die for. But whats left is a catalogue of music that I never knew were possible, I speak with a passion of a decade of hearing music that will live with me forever, not only that but I know that I still have so much more to discover, its such an exciting position to be in.
As for broadening your horizons, I have found that Soul Source and its members are the most knowledgeable and informative (and mostly friendly!) bunch you will come across, often defensive and protective of their art but it all stems from a passion that personally can only be stirred from good old fashioned soul (Northern or not) music.
Don't feel disheartened that you weren't there at the beginning, revel in the fact you are a few songs away from hearing your favourite song of all time...
...until the next one comes along.
Here's one of mine, Dave.