Thornethorpe is loosely based (geographically at least) on my home town of Ossett, in West Yorkshire. Ossett is part of the “five towns” that surround Wakefield, and each has a subtly different accent.
I have avoided typing dialogue phonetically where possible, because I find it really tiring to read (I’m looking at you, Emily Bronte) so you’ll have to imagine the accent in. There is a very thorough and rather academic look at yammer (speech) on wikipedia, but for the accent of Ossett/Thornethorpe the main thing you need to know is that we generally drop our aitches, our vowels are flat, and where possible we avoid the word “the”. The stereotypical Yorkshireman says “There’s trouble at t’mill”, we would say “There’s trouble at mill.” (This is an “incredibly complex phonetic process” according to the British Library.) There’s an adorable video of a little Yorkshire girl talking about her dad if you want to get an idea. The rock band The Cribs come from nearby Netherton, but they’ve gone a bit posh in my opinion.
The dialect of the area isn’t particularly broad, especially compared to parts of south Yorkshire, but there are a few words that might confuse international readers.
There’s not many unusual words used in the book, the two that sick out to me are “ginnel” (an alleyway between houses) and “laik” (to play – from the old Norse). Other unusual uses of words are “right” to mean “very” (“I’m right pleased”); using “us” in place of “me”, and “me” (pronounced “mi”) in place of my (“pass us me pen”); and (not in the book unless I mis-typed) switching of “was” and “were”. If there’s any bit of dialogue that doesn’t make sense then drop me a line, if there’s enough I’ll do a glossary.
*I have never heard anyone use “yammer” for “talk” in seriousness. People do say “Stop yammering on” though.