Let's consider Europe as roughly a 10 million square km area. The probability of a single point falling within a 50 km wide band (assuming the band runs the full length of Europe) is about 0.01581 (1.581%). The probability of seven points aligning within a 50 km wide band across Europe is approximately: 10^-14
To remove that constraint, so it's just any band, I think it should be more like: given a cathedral is in a particular place, how likely is it that six other cathedrals fall in a 50km wide band aross Europe.
I think if you set the probability of the first one at 1, then the rest works perfectly at any angle of band. I could be wrong, but intuitively that seems correct.
Also, the probability of the first 2 points will always be 100% because they define the line.
Also, it's not 7 random points, it's 7 of thousands of random points.
Ah - I missed this one.
But it turns out that we don't even have to go that far. The line found in TFA is about ~50km wide. Any given 50km wide line covers approximately 1% of europe's area. There are allegedly 800+ locations dedicated to st michael in the UK, so let's make a conservative estimate and say there are ~1000 in all of europe. This means that in any given 1% of europe's area, there are on average 10. Therefore literally any 50km wide line that crosses a substantial portion of europe has a solid chance at having 7 or more st michael dedications in it.
Edit: actually via a generalization of the pigeonhole principle, if we assume that europe is 3000km tall and contains 1000 sites dedicated to st michael, there must exist at least one 50km band that contains at least 17 sites.