"The link with the well is a good signpost"

And also the fact that the Worm was first caught from in the Wear. Also the proximity of Worm Hill to the Wear, and the fact that Worm's Rock (wherever it may be!) is in the middle of the Wear. There's also the way that the Worm can't be cut in half because it'll just join back together again, too. Much like if you tried to cut a river in half with a sword.

"I'd still like to know why it's Pensher Hill that's got the earthworks and not Worm Hill."

I must admit this somewhat perplexes me. Although the possibility of Worm Hill being an artificial mound should not be forgotten. Perhaps both were significant (although I have to say that the name Pensher suggests a different deity to my mind, although I'll refrain from opening that can of Worms, so to speak, at this time).