My old Dad had a strong 'Brummie' Accent, which in truth, was probably an 'Astonian' accent, as he was born in 1906, 7 years before the village became part of Birmingham.
So maybe Walliams will learn, for instance, that the accent in the North of Birmingham is different than in the South. And of course there is always the 'Grammar School' version, that was a result of colloquialisms being knocked out of pupils by teachers.
Eg. Gerrout thee 'orseroad, or yoh'll finish up in 'orspittle.
Also, I think a lot depends on whether or not you ever moved away from Brum, for long periods, in your lifetime. I went into the Royal Navy in 1954. When I finished my service ten years later, I came back to Birmingham, and was often asked where I came from. Someone even thought I was South African!
Now sidown, mekyerself a tum, an' a'summertate.
S'cuse me while I goo an' put the lid on the miskin.