I've found the late 19th Century six-inch sheets indispensable in Coed y Brenin - just that extra bit of on the ground detail that can still be found hidden among the trees..... newer maps are less helpful in this respect.
Except when you're looking for something that may have happened in the period after the 19th C which is what I was in search of, but again the 1940's map suggested that a nearby smelt mill was still largely still complete then, it's just a ruin now with some outlines of walls, so I'm not that sure that newer maps always have a limited use.
I was actually more interested to find out how long that mapping has been available on the site as I hadn't noticed it before (unlike the 19th C/pre-WW1 stuff, which as you say is a very useful source), it doesn't cover Wales yet so it's possible it may reveal a few nuggets for you then:flowers:
Jim