> when the true, magnetic and grid norths met in the village of Langton Matravers in Dorset
Love that, sounds like something Douglas Adams or Terry Pratchett would write :)
Langton is obviously "long town", but Matravers is a very strange non-English sounding name, and indeed according to Wikipedia it's from French: https://en.wikipedia.org/wiki/Langton_Matravers#History
Langton Matravers is about 2km north of the coast. The 3 norths will have met land at a place called Dancing Ledge, at about SZ 00000 76833 (50.59121, -2.00000).
https://en.wikipedia.org/wiki/Dancing_Ledge
The quarry caves at Winspit are worth an explore if you're in the area - they've been used in the set for Dr Who, Blake's 7 and Andor.
https://en.wikipedia.org/wiki/Winspit