Well the thing that strikes me is how much variation in what gets played in sessions in different parts of the country now.
There seem to be some tunes that are recognised everywhere (and usually fought over as to where they came from!), but in each locality you will find tunes that are regullary played there, but are not so often played elsewhere.
When we moved from Somerset to Lancaster I was quite expecting to find some differences, but I find the same level of differences within 30 miles.
What does not quite follow is that the tunes played in each of the sessions are not all of the same regional origin, but different tunes from the same location have travelled in different directions.