Brown's is ok. I went in last autumn and met the "caretaker". The Muddy Hole is some where in the East of the survey around the centre of the East side. To the right of the word Steps and to the left of Old Level (quarry).
If I were you, I'd drive a few miles further and do something else.
I think the SMCC used to provide a survey add on that showed MH and its relation to the rest of their survey...
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