Since they’re not included in the vocabulary, I can only give a phonetic spelling:
GAY-thoch (which I’m assuming means “did you have…?” plural or polite)
and NAY-thoch (which appears to mean “did you…?” plural or polite)
How are these actually spelled? Am I even hearing them correctly? And is there a good book with verb conjugations if I fall over things like this again?
gaethoch chi
naethoch chi
Your meanings are correct, though they can be statements, as well as questions i.e. you got/had, you did
(Section 296 in my second edition of Gareth King’s Modern Welsh book)
The biggest of diolchs! Also for noting the book and section in which the information can be found, as well as the info on statements. 
As well as Gareth’s excellent grammar books (which of course give you so much more than just verb conjugations 
), there is this one -
http://www.gwales.com/bibliographic/?isbn=9781859021385&tsid=10#top
Ah: perffaith! Diolch!
Oh, are they not in the vocab? They should be… they pitch up for the first time in Challenge 2?
Yep, they show up initially in Challenge 2. And no, they’re not in the vocabulary… nor are several other words. It just so happens that I know the dach chi form which was used a few times, but I’m sure a lot of people don’t.
I’m also guessing at (going phonetic here!) “gofyn ihdd-in nhw.” Am I anywhere near right? That’s also not in the vocab. What’s the word?
A word not being given in the vocabulary was less of a problem in Level 1 because both the explanation and speaking rates were quite a bit slower. Now they’ve picked up substantially, so it’s harder to discern contractions, elisions, and "what-the-heck-was-that?!"s.
But it’s sure giving my ears a workout
gofyn iddyn nhw - to ask them. The i after gofyn conjugates (“i ti” but “iddo fo”, etc), and iddyn is simply the conjugation for ‘nhw’.
I was thinking this myself, and thanks for confirming it Siaron! I’m on Level 2 south and have seen iddyn nhw quite and came to the conclusion it was being used the same as “i ti” or “i fi”. For example, is the following correct: “dw i’n moyn i ti siarad gyda fi” and “dw i’n moyn iddyn nhw siarad i fi”?. It’s amazing how the SSiW process just makes you see how to use things naturally.
yup, spot on 
Fixed those two. If you spot any others, I’d love to hear… 
Bless you, sir!
I know what you mean. I knew how it was being used; I just wasn’t sure what the word was.