"coiner" - one who counterfeits coins. Not something we hear of much today, but in earlier times was a reasonably familiar sort of criminal.
"box-room" - a room in a house (usually a small room) which is just used for storing things, usually things that are not wanted very often. The collocation "box-room attic" is unusual, because these two words mean nearly the same thing. (Not quite, because a box-room could be anywhere in a house, not just under the roof).
"cistern": yes, it is still quite common for British houses to have a water-tank in the roof space - sometimes two (hot and cold). This was partly to provide a head of pressure for taps and showers, and partly to guard against interruptions in supply (and in the case of hot tanks, to provide a reservoir of heated water for when you needed it quickly, as in a bath).
There is not a literal tunnel, it is saying that between the tank and the sloping roof there is a long and narrow dark place (dark because it is behind the cistern).
"of course" is a parenthetical remark meaning "as you already know", or "as is obvious". It is much more common in speech than in writing, but of course this writing is meant to suggest somebody is speaking, and telling a story.
"For" says that the sentence is a reason or explanation of what precedes. So the meaning of "For of course he was thinking .. " is something like "(He was excited) because, as you probably realise, he was thinking ... "
Yep, it's perfectly grammatical and idiomatic. I'm flattered that I was invited ~ I'm flattered to have been invited, both absolutely native. Of course, this alternation is only possible when the subject of flatter is also the subject of the embedded clause. In Brown's case, the subjects were different (I versus you). So, she could only have said, as she did, I'm flattered that you're so interested in my vagina. The nearest alternative, My vagina is flattered that you are interested in it (her?) might have sounded facetious.
Best Answer
This could make for some entertaining talk, like "Who's On First" but is not the right way to put it.
The "next" of "next house but one" cannot be reliably used outside of the phrase itself unless you are hoping to confuse the carpenter.
Also putting it as a modifier to bigger or smaller confounds the intended meaning. The phrases "next tool but one" allow a phrase to be more compact. These are less and less useful the more complex you meaning gets.